200 DISCOVERY REPORTS 



The larva has now reached its maximum development, and in the next stage it has all 

 the characters of a young Cephalodisais; the alimentary canal is U-shaped and the pro- 

 boscis shifts towards the ventral aspect. This transformation, since it is accompanied by 

 so many changes, can be regarded as a metamorphosis. 



Plate XLIII, fig. 4, is a median longitudinal section of a young Cephalodisais which 

 shows almost all the characters of the adult. Though the alimentary canal is U-shaped, it 

 is a uniform tube, which is not differentiated into pharynx, stomach and intestine. The 

 postero-dorsal half of the tube is slightly wider and a lumen is clearly seen, but the 

 entire wall of the alimentary canal is still formed of cells which are similar in shape and 

 structure. Behind the mouth two diverticula are developed from the dorsal wall of the 

 pharynx. The first diverticulum, which is slightly longer than the other, tapers slightly, 

 and is directed forwards between the right and left collar cavities into the base of the 

 proboscis cavity. This gives rise to the notochord, though at this stage it is formed of 

 cells which are similar to those of the alimentary canal. The second diverticulum is 

 much smaller and is directed dorsalwards. It touches the dorsal body wall at the pos- 

 terior end of the brain and is called the pharyngeal diverticulum. As the section passes 

 through the median line the paired arms and the collar cavities are not seen. The pro- 

 boscis cavity is much compressed, but the ventral part of the trunk cavity is very 

 spacious and extends posteriorly into a small sac-like part which marks the origin of the 

 stolon. 



Plate XLIV, figs, i , 2, are transverse sections of the collar and trunk regions of a young 

 individual slightly older than the one represented in the previous figure. These show 

 more details of structure, especially of the coelomic cavities. The notochord is situated 

 between the collar cavities and the proboscis. Its lumen is comparatively wide and is 

 filled with a deeply staining substance. Nuclei and cell limits are clearly visible in its wall. 



The mesodermal cells lining the proboscis and collar cavities have multiplied con- 

 siderably, so that they not only form the thin lining epithelium of the coelom but fill the 

 greater part of the cavities. In the proboscis cavity they are aggregated into two 

 prominent clusters on either side of the notochord, and from these clusters protoplasmic 

 strands extend towards the ventral wall. These give rise to the bundles of transverse 

 muscles of the proboscis, which radiate from the septum between the proboscis and the 

 collar cavities (John, 1931, p. 250). 



The ectoderm of the trunk is very thick and carries large numbers of refractive beads. 

 The cellular structure of the alimentary canal and the mesoderm is very distinct 

 (Plate XLIV, fig. 2). The stomach is a thick-walled tube with a narrow lumen. The 

 coelomic epithelium forms a continuous lining of the body cavities. On the dorsal side, 

 the right and left body cavities do not meet, as the intestine is pressed closely against the 

 dorsal body wall ; but ventrally the two cavities become slightly wider and meet along the 

 median line, giving rise to the ventral mesentery. The enlarged ventral part of the trunk 

 coelom, which has already been referred to, is filled with large numbers of mesodermal 

 cells. These give rise to the muscles of the stalk and the postero-ventral longitudinal 

 muscles. 



