The larva has now the characters of a trochosphere, but differs from the typical trochosphere in the post-oral part of the ciliated equatorial ring being more important than the prae-oral, and in the absence of an anus.
It firstly differs from thetrochosphere larva in the character of the ciliation.
The larval form just described has been named by Lankester the trochosphere larva.
The transverse ciliated rings of the Holothurian and Crinoid larvae are of no importance in the comparison between the trochosphere larvae and the larvae of Echinodermata, since such rings are frequently secondarily developed.
It has no doubt certain resemblances to the trochosphere larva of Chaetopoda, Mollusca, etc.
Nothing like the vaso-peritoneal vesicles of the Echinoderm larvae has been found in the trochosphere; nor have the characteristic trochosphere excretory organs been found in the Echinoderm larvae.
The segmentation exhibited is of a kind which all the trochosphere larval forms seem to have been capable of acquiring.
From the trochosphere stage the larva passes into what has been called by Lankester the veliger stage (fig.
The Rotifers have many points of resemblance to the trochosphere, especially to that form of trochosphere characteristic of the Mollusca.
Some of the Gephyrea in the presence of a praeoral lobe resemble certain trochosphere types.
Tornaria is a very interesting larval form, since it is intermediate in structure between the larva of an Echinoderm and trochosphere type common to the Mollusca, Chaetopoda, etc.
A trochosphere stage with a velum but without a shell is succeeded by a veliger stage with a still more developed velum, a dorsal shell, and a ventral foot.
There is every reason to believe that the types with a Trochosphere larva, viz.
The praeoral ciliated ring of the Trochosphere is probably directly derived from the ciliated ring of Pilidium, which is itself the original ring of the prototype of all these larval forms.
Chaetopod Trochosphere than to any other larval type.
In the case of the Echinodermata the anus must have been formed on the dorsal side of this ring, and in the case of the Trochosphere on the ventral side; and so the difference in position between the two rings was brought about.
Larval form from which the Trochosphere larva may have been derived.
In development some Lamellibranchia pass through a free-swimming trochosphere stage with pre-oral ciliated band; other fresh-water forms which carry the young in brood-pouches formed by the ctenidia have suppressed this larval phase.
The early larva of Anodonta is not unlike the trochosphere of other Lamellibranchs, but the mouth is wanting.
In Annelids with trochosphere larvae a certain amount of mesenchyme is formed at an earlier stage and gives rise to the muscular bands of the young larva.
Embryo of Limnaeus stagnalis, at a stage when the Trochosphere is developing foot and shell-gland and becoming a Veliger, seen as a transparent object under slight pressure.
B, The diblastula has become a trochosphere by the development of the ciliated ring vr (optical section).
The foot now protrudes below the mouth, and the post-oral hemisphere of the trochosphere grows more rapidly then the anterior or velar area.
But in these epibolic forms, just as in the embolic Paludina, the embryo proceeds to develop its ciliated band and shell-gland, passing through the earlier condition of a trochosphere to that of the veliger.
Aplysia, like other Mollusca, develops a primitive shell-sac in its trochosphere stage of development, which disappears and is succeeded by a nautiloid shell (fig.
A well-marked trochosphere is formed by the development of an equatorial ciliated band; and subsequently, by the disproportionate growth of the lower hemisphere, the trochosphere becomes a veliger.
C, Side view of the trochosphere with commencing formation of the foot.
E, The trochosphere passing to the veliger stage, dorsal view showing the formation of the primitive shell-sac.
It will be seen, therefore, that the range of the Trochosphere larva covers a large portion of the ground occupied by our Grade IV.
Are we to suppose that animals which possess a Trochosphere larva are all descended from one common ancestor?
When we speak, however, of the occurrence of the Trochosphere throughout a wide range of animal life, it must be understood that its presence is not necessarily uniform throughout a group in which it occurs.
The Trochosphere larva is, of course, only adapted for aquatic existence, and is necessarily absent in the case of terrestrial forms.
The main characters of theTrochosphere are, however, the same in very widely differing groups.
Or are we to think that the Trochosphere is a form of body very convenient for the necessities of juvenile existence in the sea, and therefore independently evolved by animals which are not directly related to each other?
A very wide-spread form of larva, more advanced in its structure than the little Gastrula that has been already named, has received the name of Trochosphere or Wheel-ball (Fig.
A, Embryo, and B, Young Trochosphere Larva of the Lamellibranch Teredo.
Young Trochosphere Larva of the Gephyrean Echiurus, seen in optical section.
This typical structure of the larva is often departed from, and the molluscan trochosphere can be distinguished from the annelidan by the possession of a rudiment at least of the shell-gland and foot (figs.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "trochosphere" in a variety of sentences. We hope that you will now be able to make sentences using this word.
