80 



INVERTEBEATA 



CHAP. 



couples. Of these, one couple, which project into the post -oral 

 prominence of the larva, are known as the dorsal couple ; then follow 

 dorso-lateral, ventro-lateral and ventral couples. The ventro- 

 lateral couple develop more quickly than the others, they soon reach 

 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 the 

 aboral pole. 



The chambers into which the 

 coelenteron is divided by the 

 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, making 

 eight 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 Urticina 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 way ; 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 

 (Eig. 61). We may note that the taeniolae of the Hydra-tuba 

 larva become 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- 



FiG. 60, — Two transverse sections through 

 a larva of Urticina crassicornis to show 

 the formation of mesenteries. (After 

 Appelldf ) 



A, section through post-oral region of larva. 

 B, section through middle of larva, d.c, dorsal 

 couplo of mesenteries ; d,l, dorso-lateral mesen- 

 teries ; Jil, mesenterial filament ; v. I, ventro-lateral 

 mesenteries ; v, ventral mesenteries. 



