DEVELOPMENT OF CEPHALODISCU S 203 



from the mesodermal structures of the parent stolon, which at a later stage give rise to 

 the muscles and connective tissue of the trunk. 



The heart with the pericardium is a large thick-walled vesicle, which at this stage is 

 slightly larger than the primitive gut. The invagination of the posterior wall of the 

 vesicle giving rise to the heart is clearly seen. The large space inside the heart is filled 

 with a clear fluid, which stains deeply in haematoxylin. 



At a later stage, when the arms have begun to develop, the mouth is formed as an 

 ectodermal invagination which opens into the lumen of the primitive gut at about one- 

 third the distance from its anterior end. The region in front of the mouth opening be- 

 comes modified into the notochord, while the posterior part gives rise to the alimentary 

 canal. Fowler (1905), in describing the development of the buds in Rhabdopleura, states 

 that probably the greater part of the alimentary canal is derived from a thin-walled tube, 

 which originates from the mesentery of the stolon. He believed, however, that the 

 anterior part of the alimentary canal was derived from the ectoderm and that the notochord 

 was formed from the stomodaeal portion. Though the origin of the alimentary canal in the 

 buds of Cephalodiscus is to a great extent similar to that of Rhabdopleura, it can be 

 definitely shown that in the former group the notochord is a part of the primitive gut 

 and not a stomodaeal growth. 



DISCUSSION 

 LARVAL AND BUD DEVELOPMENT 



The differences between the development of the larva and bud of Cephalodiscus may 

 be summarized as follows : 



(i) The bud is attached to the proximal end of the stolon by a narrow stalk, the length 

 of which increases with the growth of the bud. The trunk region of the bud is a con- 

 tinuation of the distal end of the stalk, there being no discernible difference between the 

 thickness of the two during the early stages. 



(ii) The proboscis of the bud is much larger than the rest of the body and is cha- 

 racterized by the presence of the red line from a very early stage ; whereas in the larva 

 the red line is not visible and the proboscis is comparatively much smaller. 



(iii) The bud breaks off from the stolon only after it has reached its maximum de- 

 velopment, but the larva is free-swimming or crawls about independently. 



(iv) In the buds the refractive beads are found only in the tips of the arms or in the 

 dorsal wall of the proboscis ; but in the larva these are present in very large numbers in 

 the ectoderm of the trunk. If the view that these refractive beads are defensive organs 

 should prove to be correct, then it follows that their occurrence in such large numbers 

 in the body wall of the larva is sufficient evidence for the free-swimming or crawling 

 habits of the larva. These refractive beads are not present in the trunk wall of the bud 

 or of the adult zooid, as the bud is not exposed to danger and the trunk of the adult is 

 protected inside the coenoecium. 



