254 DISCOVERY REPORTS 



tracts. Fig. i passes through the anterior region of the proboscis in front of the ganghon. 

 Apart from the three layers of the body wall no trace of nerve plexus is seen in this 

 region, and this supports my opinion that the pre-oral nerve plexus does not pass round 

 the anterior tip of the proboscis. Harmer (1905, p. 72) also doubted the existence of 

 such a nerve, though he believed the nerve plexus at the base of the glandular region of 

 the ventral wall to be connected with the ganglion by a nerve which passed through the 

 tip. Fig. 2 shows the region of the proboscis in front of the notochord. The proboscis 

 cavity is very wide and contains inside it the proboscis canals and the heart and peri- 

 cardium. Dorsal to the heart is the thick pre-oral nerve plexus which spreads out laterally 

 on each side and becomes continuous with the nerve plexus of the ventral wall. Fig. 3 

 shows the ganglion, and, ventral to this, the heart and pericardium, the two proboscis 

 pores and the anterior horns of the right and left collar cavities. The lateral plexus is 

 more prominent and is seen extending to the sides. Fig. 4 shows the continuation of 

 the ganglion in the dorsal wall of the collar from which nerves are given off to the arms. 

 These nerves pass along the dorsal wall of the axis of the arms, and at the base of each 

 arm spread out laterally fomiing a complete ring of nerve plexus. Fig. 5 shows the tip 

 of the dorsal diverticulum of the pharynx and the origin of the main lateral post-oral 

 nerves. These nerves proceed downwards along the right and left sides of the pharynx 

 and in fig. 6 they are seen at the base of the post-oral lamella below the right and left 

 collar pores. They give off three branches on each side, the first (In^) proceeds to the 

 wall of the pharynx, the second (/«2) passes along the posterior wall of the post-oral 

 lamella, and the third (///g) goes to the ventro-lateral body wall. By the time fig. 8 is 

 reached the lateral nerves become so small that they cannot be easily distinguished from 

 the surrounding tissue. Beyond the region of the gill opening no distinct nerve tract 

 is visible, and I was not able to discover the nerves in the stolon nor any sign of the 

 lateral nerves extending into it. 



In conclusion it may be pointed out that the pre-oral and lateral plexus together form 

 a continuous plate of nerve plexus which spreads out from the anterior and lateral sides 

 of the main ganglion in the dorsal wall of the proboscis. In the present account this 

 is called the pre-oral nerve plexus. The paired lateral nerves and the "median pre-oral" 

 nerve of Masterman are not distinct branches, but are only parts of this continuous 

 plate of nerve plexus which in individual longitudinal and transverse sections appear 

 like branches. The paired post-oral nerves do not form a complete ring but break up 

 into branches at the base of the right and left collar pores. 



CLASSIFICATION OF THE SPECIES OF CEPHALODISCUS 

 WITH A LIST OF KNOWN FORMS 



Ridewood (19 18 a) gave a synopsis of the species of Cephalodiscus and a list of all 

 recorded specimens in his contribution to the 'Terra Nova' reports. The present ex- 

 pedition has brought to light two new species. I therefore feel justified in revising the 

 classification especially in view of the fact that a new sub-genus Acoelothecia had to be 



