Body with an anterior region of fifteen setigerous somites separated from a larger posterior region by a specialized somite, the sixteenth.
Branchiae two pairs; branched; attached on somites II and III.
Anterior somites of thorax darkened above, down the sides on both sides of the tori and also more or less ventrally with purplish brown pigment.
The type is incomplete caudally, at present consisting of ninety-five somites and having a length of 35 mm.
Peristomium shorter than prostomium and than the next twosomites combined; divided by a transverse furrow.
In the type the posterior thoracic somites are twice or more as long as wide, but in some paratypes the relative length is much less.
Unlike those of alata, somites II and III are not confounded laterally but are distinct throughout.
The brain innervates the eyes and feelers, and must be regarded as a "syncerebrum" representing the ganglia of the three foremost limb-bearing somites united with the primitive cephalic lobes.
The head of an insect carries usually four pairs of conspicuous appendages--feelers, mandibles and two pairs of maxillae, so that the presence of four primitive somites is immediately evident.
The number of limb-bearing somitesin the insectan head is thus seen to be seven.
Besides the five limb-bearing somites just enumerated, two others must now be recognized in the head.
The Origin of the Vertebrate Body-cavity and Excretory Organs: the Meaning of the Somites of the Trunk and of the Ductless Glands," vol.
We see how the first of the trunk somites is formed just posteriorly to the head region, and then more and more somites are formed by the addition of fresh segments posteriorly to the one first formed.
The Arthropoda are all distinguished from the Chaetopoda by the fact that the head consists of one or more somites which lie in front of the mouth (now called prosthomeres), as well as of one or more somites behind it (opisthomeres).
At most they are present on all the pedigerous somites excepting the first and the last.
This gives sevensomites to the Hexapod's head, the tergites of which are fused to form a cephalic carapace or box.
The appendages of four or more additional following somitesmay be turned upwards towards the mouth and assist in the taking of food.
Rarely only one, and usually at least two, of the somites following the mandibular somite carry appendages modified as jaws (with exceptions of a secondary origin).
According to older views the increase of the number of somites in front of the mouth would have been regarded as a case of intercalation by new somite-budding of new prae-oral somites in the series.
In Crustacea the fourth or mandibular somite never has less than the two following somites associated with it by the adaptation of their appendages as jaws, and the ankylosis of their terga with that of the prosthomeres.
The carapace which covers the upper side of the head and thorax is not formed, as might be thought, simply by the terga of the somitesbecoming soldered together.
In these points they resemble the shrimp-like forms, but there is no carapace, and all the somites of the thorax are distinct, so that the form of the body is rather that of an Amphipod or Isopod.
The abdominal somiteshave a row of spines down the middle of the back, and the telson has a forked shape.
All the other somites of the body are distinct (in some Isopods, however, the abdominal somites are coalesced), but the telson is not separate from the last somite.
At the sides the somites are connected together by hinge-joints, which allow them to move only in a vertical plane.
In the Lobster this fold has become adherent to the thoracic somites down the middle of the back, but at the sides it hangs free, enclosing on each side a cavity within which lie the gills.
On the upper side of the abdomen the terga of the somites overlap, the front part of each being pushed under the tergum in front when the abdomen is straightened, and only exposed to view when the abdomen is bent.
If one of the somites of the abdomen be separated from the others, it will be seen (Fig.
They have a small carapace, which only involves the first two thoracic somites, the rest of the somites being distinct.
Each of the free somites and of those forming the tail-shield bears a pair of two-branched appendages, not differing greatly from the posterior appendages of the head, but becoming smaller and more flattened towards the hinder end of the body.
The somites following the second are formed in the regular manner, from before backwards, out of the unsegmented posterior part of the embryo, which rapidly grows in length to supply the necessary material (fig.
The general history of the embryo has already been traced up to the formation of the first formed mesoblastic somites (fig.
They are the foremost somites of the trunk, and those behind them continue to be added, like the segments in Chaetopods, between the last formed somite and the end of the body.
In the Chaetopoda and the Tracheata the body cavity arises as a series of paired compartments in the somites of mesoblast (fig.
After the separation of the vertebral bodies from the somites the remaining parts of the somites may be called muscle-plates; since they become directly converted into the whole voluntary muscular system of the trunk (fig.
The medullary groove behind the region of the somitesdilates into an embryonic sinus rhomboidalis like that of the Bird.
The later development of the somites has not been worked out with the requisite care, but it would seem that they form somewhat cubical bodies in which all trace of the primitive slit is lost.
In the somites the cavities become eventually obliterated, and from their inner sides plates of tissue for the vertebral bodies (fig.
The mesoblastic somites begin to be formed in the lateral plates of the mesoblast before the closure of the medullary folds.
The somites are first formed in the neck, and are added successively behind in the unsegmented posterior region of the embryo.
The extension of the somites to the anterior end of the body in Amphioxus clearly proves that somites, similar to those of the trunk, were originally present in a region, which in the higher Vertebrata has become differentiated into the head.
The embryo is considerably longer, and a great number of mesoblastic somites are visible.
At the sides of the trunk are seen the mesoblastic somites (p.
One of a series of flat, fringed, and usually bilobed, appendages, of which several pairs occur on the abdominal somites of many crustaceans.
The ventral part of any one of the somites of an arthropod.
The medullary folds and notochord are evident at this stage, but no mesoblastic somites are to be seen.
Figure 3 is a dorsal view of the next stage to be described; about fifteen pairs of somites are present.
The external boundaries of the somites are not distinct.
Embryo with about thirteen mesoblastic somites in which the flexure of the hind part of the body has commenced.
In Crustacea mesoblastic somiteshave not usually been found, though they appear occasionally to occur, e.
The somites have increased to five, and there are indications of a sixth being budded off from the posterior mass of opaque tissue.
The mesoblastic somites do not extend outwards beyond the edge of the ventral plate, and the corresponding mesoblastic somites of the two sides do not nearly meet in the middle line.
The anterior pair of mesoblastic somites gives rise to the swellings of the praeoral lobes, and to the mesoblast of the head[573].
In all the sections the epiblast lying over the somites is thickened, while elsewhere it is formed of only one layer of cells; and this thickening subsequently appears to give rise to the nervous system.
Behind the six somites with prominent appendages there are four well-marked somites, each with a small protuberance.
We cannot speak positively as to how they come there, whether by a pushing forward of the anterior somites of the previous stage, or by the formation of new somites anteriorly to those of the previous stage.
One of the somites (arthromeres) which make up the head of arthropods.
One of the jointed antenniform appendages of the posterior somites of certain insects.
The cavities of all thesomites unite to form a common cavity, the cœlom or perivisceral space of the Cockroach.
In the adult female Cockroach, however, the 8th and 9th somites are telescoped into the 7th, and completely hidden by it.
By the telescoping of the 8th and 9th somites the sterna take the position shown in fig.
The walking legs are articulated to the inferior portion of the pleural or lateral area of the somites close to the external margins of the sterna, which widely separate those of the left from those of the right side.
The only exception to this is presented by Scutigera, where the terga corresponding to the somites bearing the fifteen pairs of legs are reduced by fusion and suppression to seven.
In the case of the somites bearing the walking legs the tergal and sternal elements are preserved without fusion with the corresponding plates of the preceding or succeeding somites, so that great flexibility of the body is retained.
The structure of the gonopods is unknown, and the homology between the two valves and the skeletal elements of the somites in question not clearly understood.
The sternal plates of all the jaw-bearing somites have disappeared, except in the case of the somite of the toxicognath, where it may be vestigial.
In Craterostigmus the genital and anal somites are represented by a pair of elongate valves projecting between the legs of the last pair.
The genital and anal somites are not retractile within the last leg-bearing somite, and the gonopods typically persist in the male as small two-jointed appendages and in the female as jointed or unjointed sclerites.
The tergal elements of the somites bearing the antennae, mandibles and maxillae appear to be represented by the head-shield or cephalite.
Chilopods differing principally from the Geophilomorpha in that the number of leg-bearing somites is definitely fixed at twenty-three or twenty-one.
Spiracles are present upon all the leg-bearing somites except the first and last; and the legs which are short and subequal in length consist of six segments, the basal of which remains small.
Defn: One of a series of flat, fringed, and usually bilobed, appendages, of which several pairs occur on the abdominal somites of many crustaceans.
Defn: The ventral part of any one of the somites of an arthropod.
Only seven of the eight thoracic somites are visible, the first being fused with the cephalon.
It is quite probable, however, that in the primitive ancestors of existing Crustacea a still smaller number of somites formed the head.
Throughout the whole of the Malacostraca the thorax consists of eight and the abdomen of six somites (fig.
In the Isopoda and Amphipoda, where, as a rule, all the thoracic somites except the first are distinct (fig.
At successive moults the somites increase in number, newsomites being added behind those already differentiated, from a formative zone in front of the telsonic region.
The numbers appended to the somites do not correspond to the enumeration adopted in the text.
The appendages posterior to the mandibles appear as buds on the ventral surface of the somites, and in the most primitive cases they become differentiated, like the somites which bear them, in regular order from before backwards.
The large number and the uniformity of the trunk somites and their appendages, and the structure of the nervous system and of the heart in Apus, are Annelidan characters which can hardly be without significance.
For practical purposes, however, it is convenient to include the two followingsomites also as cephalic.
Fourthly, there is a tendency to form a pygidial or telsonic shield, "a fusion of the posterior somites of the body, which is precisely identical in character with the metasomatic carapace of Limulus.
It is, at present, impossible to demonstrate the actual number of somites in the cephalon of the trilobite, but I believe that Beecher was correct in holding that the glabella was composed of four segments.
Now, this Divine character is native to the sons of God.
The three short clauses of the citation supply, in effect, a threefold measure of the gifts of Christ to His Church.
All this was true in parable of the dispensation of grace to mankind through Jesus Christ; and His ascension disclosed the deeper import of the words of the ancient Scripture.
This view does not appear to me quite satisfactory, since on the analogy of Spiders and other Arthropoda the fresh somites ought to be added by a continuous segmentation of the posterior lobe.
The mesoblastic somites for these lobes are established somewhat later than for the true segments, but only differ from them in the fact that the somites of the two sides are united by a median bridge of undivided mesoblast.
The mesoblastic masses of the head are probably not to be regarded as forming a pair of somites equivalent to those in the trunk, but as forming the mesoblastic part of the prae-oral lobe, of which so much has been said in the preceding pages.
The first important change takes place when about six somites are established; the mesoblast then becomes divided into two lateral bands, shewn in section in fig.
B divided into splanchnic and somatic layers, and in the abdomen at any rate into somites continuous with those of the ventral part of the mesoblast.
Not only is a pair of mesoblastic somites formed for each segment of the body, but also for the procephalic lobes (fig.
Since the cavity of the mesoblastic somites is part of the body cavity, all the appendages contain prolongations of the body cavity.
In the trunk however the division into somites precedes the horizontal splitting of the mesoblast.
The cavities in the separate mesoblastic somites also cease to be distinctly circumscribed.
The somites formed by the segmentation gradually grow upwards and meet in the dorsal line.
It becomes raised into a series of transverse rings which bear no relation to the true somites of the mesoblast.
The splanchnic layer of the trunk somites gives rise to the muscular and connective-tissue wall of the mesenteron, and also to the walls of the vascular trunks.
There are a number of other important points of structure besides those referring to the somitesand appendages in which Limulus agrees with Scorpio or other Arachnida and differs from other Arthropoda.
Opisthosoma when segmented showing the same number ofsomites as in the Pedipalpi; usually unsegmented, the prae-genital somite constricted to form the waist; the appendages of its 3rd and 4th somites retained as spinning mammillae.
The possibility of another interpretation of the anterior somites of the mesosoma and the prae-genital somite must be borne in mind.
In none of them are the appendages known, but in the form of the two carapaces and the presence of free somites they are distinctly intermediate between Limulus and the Trilobitae.
How many somites are there from the anterior end to the girdle?
Prothorax, mesothorax, and metathorax, the three divisions or somites into which the thorax of any insect is divided.
Identify also the somites and the girdle, the mouth with its projecting lip, and the anal opening.
How many somites are there in a crayfish's body, if each somite bears only one pair of appendages, as many scientists believe?
To which somitesof the thorax are the wings attached?
As the upper edges of two opposing somites grow together over the medullary tube from right and left, the vertebra-arch becomes closed.
This vesicular condition of the provertebra is of the greatest phylogenetic interest; we must, according to the coelom theory, regard it as an hereditary reproduction of the hollow dorsalsomites of the amphioxus (Figures 1.
The number of the metamera, and of the embryonic somites or primitive segments from which they develop, varies considerably in the vertebrates, according as the hind part of the body is short or is lengthened by a tail.
In the incubated hen's egg the first somites make their appearance thirty hours after incubation begins (Figure 1.
Among the mammals the embryos of the marsupials have three pairs of somites (Figure 1.
They develop more slowly in the embryo of the rabbit; this has three somites on the eighth day (Figure 1.
It has, however, been alleged that this cavity is formed by a pair of mesoblastic somites (N.
TER'GITE, the tergum or back of one of the somites or segments of an arthropod, &c.
They have a carapace, covering and uniting the somites of the head and thorax and inclosing a gill chamber on each side, and usually have five (rarely six) pairs of legs.
It represents, however, only a very small part of the primitive straight gut, corresponding to not more than two or three somites of the embryo.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "somites" in a variety of sentences. We hope that you will now be able to make sentences using this word.
