C, taken from van Wijhe, and shows how the segmental duct is left in the remains of the atrial cavity.
Kowalewsky described this atrial space as formed by the ventral downgrowth of pleural folds on each side of the body, which met in the mid-ventral line and enclosed the branchial portion of the gut.
It is in this very space, close against the gill-slits, that the thymus glands of Ammocoetes are found, in the very place where the nephric tubules of Amphioxus would be found if its atrial cavity were closed completely.
Boveri considers that the obliteration of the atrial cavity in the higher vertebrates is not complete, but that its presence is still visible in the shape of the pronephric duct.
The gills lead also, as in Ascidians, to another cavity, theAtrial chamber.
These form a kind of basket-work, consisting of minute holes with intermediate supports; and they are associated with a special cavity outside them called the Atrial chamber.
The atrial cavity, from its mode of origin as a pair of epiblastic involutions[10], is clearly a structure of the same nature as the branchial or atrial cavity of Amphioxus; and has nothing whatever to do with the true body cavity.
The embryo becomes hatched not long after the formation of the oral and atrial openings, and the five epiblastic processes undergo atrophy.
In the larva it is provided with only two ciliated slits, which open into the, at this stage, pairedatrial cavity (fig.
Into the former the single atrial invagination opens.
This sack next becomes a prominence in the atrial cavity, and eventually constitutes a brood-pouch.
In a later stage the openings of the two atrial tubes gradually travel backwards, and at the same time approximate, till finally they meet and coalesce at the posterior end of the blastoderm behind the nervous disc (fig.
In Amphioxus they pass by dehiscence into the atrial cavity, and thence through the gill slits and by the mouth, or by the abdominal pore (?
The branchial cavity is thus thrown into communication with the atrium, and the upper surface of the latter now unites with the outer tunic, and the external atrial opening is formed by absorption.
It has already been stated that the anus opens into the original left atrial cavity; when the two cavities coalesce the anus opens into the atrial cavity in the median dorsal line.
At the junction between them is formed an elongated ciliated slit, leading from the branchial sack into the atrial cavity of each side.
From this capsule a duct passes to the atrial cavity, which, though called the oviduct, functions as an afferent duct for the spermatozoa.
In the Cyathozooid the atrial spaces gradually atrophy, with the exception of the external opening, which becomes larger and more conspicuous.
In the asexually produced buds of Ascidians the atrial cavity appears, with the exception of the external opening, to be formed from the primitive branchial sack.
It is enveloped in a capsule continuous with a duct, which opens into the atrial cavity, and is usually spoken of as the oviduct.
After the above changes are far advanced towards completion, the openings of the two atrial sacks gradually approximate in the dorsal line, and finally coalesce to form the single atrialopening of the adult.
Behind the first pair of branchial clefts a second pair is formed during larval life by a second outgrowth of the branchial sack meeting the epiblastic atrial involutions (fig.
Although within the atrial cavity, it is separated from the latter by a narrow coelomic space, bounded towards the atrium by coelomic and atrial epithelium.
The ligamentum denticulatum is thus lined on one side by the epiblastic atrial epithelium, and on the other by mesoblastic coelomic epithelium.
These are the atrial coelomic funnels or brown funnels, so called on account of the characteristic pigmentation of their walls.
The water after passing through the branchial network is received into narrow passages and conducted to a larger cavity--the cloacal or atrial chamber.
From the atrialchamber the water flows out into the external world.
In the Botryllidae and Polystyelidae the individuals are not segmented and in the former family are arranged in star-shaped groups about a common cloaca, into which the atrial siphons of the different individuals open.
In forms like Cynthia it holds the same relative position with regard to the mouth, but by the doubling of the body (to be explained further on) it is also brought near the atrial aperture, where it is shown in our first diagram.
After passing through the throat, they come to the stomach, where digestion takes place, and then the ejectamenta are carried out through the intestine and poured into the bottom of the atrial cavity.
The atrial cavity remains open to the exterior at one point, the atrial pore (at.
Head of still later tadpole in horizontal section to show atrialchamber formed by operculum.
Compare the atrial cavity and coelom of amphioxus.
Thence they escape either by gill-slits, pharynx and mouth, or, more generally, through the atrial pore.
The two sperm ducts however open into the same distended atrial cavity before opening on to the exterior.
In Teleutoscolex there is but one pair of funnels opening into the same segment with the atrial pore.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "atrial" in a variety of sentences. We hope that you will now be able to make sentences using this word.
