Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)
Credit: Text-book of embryology / edited by Walter Heape. Source: Wellcome Collection.
Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.
116/732 (page 80)
![CHAV. the the the couples, or these, one couple, which project into tlie post-oral promiuence of the larva, are known as the dorsal couple ; then follow dorso-lateral, ventro-lateral and ventral couples. The ventro- lateral couple develop move quickly than tlie otliurs, they soon reacli the stomodaeal wall with which they fuse, and thus divide the gastric cavity into a dorsal and a ventral chamber. They also extend farther towards the aboral pole of the larva than the others, but as development proceeds all eight fuse with the stomodaeum and reach equally far towards aboral pole. The chambers into which coelenteron is divided by eight mesenteries are arranged as follows. The dorsal chamber is a median chamber at one end of the long slit-like stomodaeum; then follow a pair of dorso-lateral chambers, then a pair of lateral chambers, then a pair of ventro- lateral chambers, and finally a median ventral chamber ei<j;ht in all. When the process of formation of mesenteries is complete, per- forations take place in the septa, forming the so-called mesenterial stomata, and thus the cavities into which the coelenteron is divided by the mesenteries are placed in communication with each other. In JJrticina there are two sets of these stomata, an inner set near the stomodaeum and an outer set near the outer body wall of the larva. Both sets arise in the same w-ay; the supporting lamella becomes absorbed over a limited area and the two layers of cells which form the mesenterial fold fuse with one another, and then in the centre of this area of fusion absorption of the cytoplasm begins, and so a perforation is made (Fig. 61). We may note that the taeniolae of the Hydra-tuba larva liecome perforated in exactly the same way. After swimming for about six weeks the larvae begin to attach themselves to the substratum by the aboral pole; at first the attach- making Fig. 60.—Two transverse .sections tliroiigli a larva of Urticina crassicornis to sliow the formation of mesenteries. (After Appelliif.) A, .section tlirougli ]jost-oral region of lai v.a. B, section tlirongh middlfi of larva, d.c, dorsal couple of mesenteries ; d.l, dorso-lateral mesen- teries ; fil, mesenterial filament; v.l, ventro-lateral mesenteries ; r, ventral mesenteries.](https://iiif.wellcomecollection.org/image/b21506589_0001_0116.jp2/full/800%2C/0/default.jpg)
