DEVELOPMENT OF CEPHALODISCUS 199 



narrow as it approaches the posterior end, and ends blindly, the anus not being yet 

 formed. 



The right and left coelomic pouches are seen on each side between the archenteron 

 and the ectoderm. The right coelom has completely separated itself into three parts. The 

 proboscis coelom (Plate XLIII, fig. 2, pc) appears like a soHd rod of cells, while in the 

 collar and trunk regions the beginnings of the respective cavities are visible. On the left 

 side the coelomic pouch is still connected with the wall of the archenteron and appears like 

 a single long pouch, the two walls of which are in close contact (Plate XLIII, fig. 2, cp). 

 At its anterior end it is slightly thicker, and in this region it is connected with the wall of 

 the archenteron. This condition of the left coelomic pouch throws considerable light on its 

 possible mode of origin. Though the cells lining the walls of the coelomic pouch appear 

 to be slightly diff"erent from those of the archenteron, in the region where the two are 

 connected they are almost similar. This single pouch severs its connection from the wall 

 of the archenteron, and later the anterior third of its length is cut off by a constriction, 

 thus giving rise to the collar and trunk cavities. This condition is seen on the right side, 

 which has developed more rapidly than the left. 



It is difficult to say definitely from which side the proboscis coelom originates ; but 

 from the fact that the right side represents a more advanced stage in the development of 

 the coelom, it is probable that the mass of mesodermal cells in the proboscis is con- 

 stricted off from the anterior end of the right coelomic pouch. 



The proboscis is situated at the anterior end, and its outer wall is thick. The larva now 

 elongates slightly and the anus is formed at the posterior end. Plate XLIII, fig. 3, 

 represents a larva at this stage which has been cut slightly obliquely, so that the proboscis 

 coelom and the mouth are not quite visible. It has now an almost worm-like appearance, 

 with a prominent swelling on the dorsal side which marks the commencement of the first 

 pair of arms (Plate XLIII, fig. 3, ra). The proboscis still occupies the anterior end, and 

 part of the proboscis coelom (pc) is seen towards one side. The masses of pigment which 

 were till now uniformly distributed in the wall of the proboscis are now accumulating 

 towards the outer surface, from which they will be extruded in due time. The mouth is 

 situated ventrally and the alimentary canal is an almost straight tube with the anus at 

 the posterior end. Near the posterior end there is a small bend in the aUmentary canal, 

 which marks the commencement of the U -shaped structure of the adult. 



The body cavities are not very distinct, but part of the trunk coelom is seen on the 

 dorsal side of the alimentary canal. The dorsal part of the collar cavity extends as a wide 

 space into the rudiment of the arms, and the right and left halves extend round the 

 alimentary canal and meet on the ventral side in the small swelling, behind the mouth, 

 which represents the post-oral lamella. At this stage the larva resembles Balanoglossus. 

 It is worm-like and elongated in the antero-posterior axis. The proboscis is at the anterior 

 end and the anus at the posterior. The collar cavities surround the pharyngeal region 

 like a ring and the mouth opens ventrally; but this larva differs from the larva of 

 Balanoglossus in that the gill slits are not present and that the rudiments of the first 

 pair of arms have already appeared on the dorsal side of the collar. 



