DEVELOPMENT OF CEPHALODI SCU S 201 



The heart with the pericardium is observed for the first time at this stage. It is 

 situated at the tip of the notochord, on tlie septum between the collar and proboscis 

 cavities. Though it is formed at an earlier stage, owing to the bad state of the material 

 it has not been possible to locate its position or trace its origin ; but from its relation to 

 the collar cavities and its structure, in which it closely resembles the heart of Echino- 

 derms, e.g. Asterias rubens and Echinus miliaris, it seems probable that it originates 

 exactly as in the latter group (Narasimhamurti, 1932). 



The further development is not accompanied by any more changes. All the parts of 

 the adult zooid except the stalk are now visible. The anterior diverticulum of the pharynx, 

 which gives rise to the notochord, is still formed of cells which are undifferentiated from 

 the rest of the alimentary canal, and the muscle fibres are not fully formed. The meso- 

 dermal cells in the coelomic cavities are oval with distinct nuclei, but the protoplasm of 

 some of these cells has already commenced to elongate into strands. The stomach is a 

 thick-walled tube, which is not clearly marked off from the pharynx and intestine. The 

 gill slits and the vacuolated areas of the pharynx — the pleurochords — are not yet formed, 

 and there is only a single pair of arms. The brain is developed at the base of the dorsal 

 ectoderm of the collar and extends from the anterior end of the notochord to the tip of 

 the dorsal pharyngeal diverticulum ; but the various nerve tracts and the nerve plexus 

 in the ventral wall of the proboscis are still indistinguishable. 



In the next stage the different parts of the body are much better emphasized, but the 

 young Cephalodisciis still appears to continue its free-swimming life, if one may judge by 

 the large number of refractive beads present in the wall of the trunk. Plate XLIV, fig. 3, 

 which is a frontal section of a young zooid, illustrates the very close resemblance between 

 this stage and the adult individual. The red line of pigment and the nerve plexus 

 {pni) in the ventral wall of the proboscis have begun to appear and the mesodermal cells 

 are completely transformed into muscle fibres. The stomach has begun to assume the 

 characteristic shape of the adult and the intestine appears as a long narrow tube ; but 

 from the sections it is not certain whether the pleurochords and gill slits are yet formed. 



The structure and position of the heart is very clearly seen. It is situated on the 

 septum between the right and left collar cavities, slightly more towards the left side of 

 the median septum, and it projects into the proboscis cavity. The pericardium is an 

 empty sac, the posterior wall of which is invaginated to form the heart. The inner surface 

 of the wall of this invagination is thrown into numerous deeply staining ridges which 

 give rise to the glomeruH. 



It is not necessary to give any more details of structure, as the young Cephalodisciis 

 now resembles the adult in all essential characters, except the arms and the stalk, which 

 are still in a rudimentary state. It seems probable that while the full complement of 

 arms with the characteristic pinnules are being formed, the refractive beads in the trunk 

 disappear, the young Cephalodiscus settles down on a suitable substratum and begins to 

 secrete the coenoecium as the starting-point of a new colony. 



