476 K. RAMUNNI MENON. 



its convex side directed towards the preonil cavity. The 

 cavity of the preoral lobe is traversed by protophismic 

 strands, some of which cross it fi'om the upper wall to the 

 lower, while others run parallel to its free border. A definite 

 coelomic epithelium cannot be made out in this cavity, the 

 only structures clearly seen beneath the ectoderm being a 

 thin basement membrane (chondroid tissue), with a few 

 isolated patches of nuclei adhering to the inner face of the 

 membrane. Scattered blood-corpuscles and coelomic cor- 

 puscles are occasionally met with in this cavity, the nature 

 of which it is thus difficult to decide without embryological 

 evidence. 



Below the nerve-ganglion which lies in the root of the 

 preoral cavity, just in front of the attachment of the 

 mesentery above mentioned, there is a small space, the 

 posterior wall of which is formed by this mesentery, which 

 thus separates it from the collar cavity, and the anterior wall 

 by another mesentery stretched between the upper and lower 

 walls of the preoral lobe, which thus separate it from the 

 rest of the preoi-al cavity. This is the subneural sinus, and 

 the nerve-ganglion forms part of its roof. It is a completely 

 closed vesicle, and the epithelium forming its wall can be 

 traced along the roof of the sinus immediately below the 

 nerve-ganglion. The nerve-gauglion thus really lies outside 

 the cavity. There are two muscle strands developed in con- 

 nection with the wall of this cavity, and attached to the 

 dorsal body-wall near the ganglion. The subneural sinus 

 does not communicate with the dorsal blood-vessel; nor does 

 it contain any blood-corpuscles. Masterman compares this 

 sinus to the central blood-sinus of the proboscis of the 

 Enteropneusta. If a comparison is to be made between it 

 and any structure in the Enteropneusta, its complete epi- 

 thelial wall, and the muscular development in connection 

 with its wall, strangely support Harmer's view that it is the 

 " heart vesicle." 



The collar cavity is bounded posteriorly by the mesentery 

 between it and the trunk cavity, which is attached to the 



