BUDDING PROCESSES OF CEPHALODISCUS DODECALOPHUS. 509 



position in a section (plate 25, fig. 16) of the whole auimal (5), and they form a very 

 marked peri-pharyngeal band around the buccal chamber. Their position is readily 

 determined, in cross sections, by the fact that they are situated exactly internal to the 

 line of junction of the mesentery between collar- and trunk-cavities, with the gut wall 

 (see fig. 93). 



In fig. 94 the pleurochords form the main lateral walls of the pharynx, but the two 

 pairs of grooves are still conspicuous. 



Fig. 95 is posterior to the mouth-opening and behind the pharyngeal cleft. Here 

 the two ventral grooves, forming a peri-pharyngeal band, have united behind the mouth 

 in a medium ventral groove, with the middle line slightly raised in a ridge. The two 

 dorsal grooves are still separated by a very characteristic median ridge of thickened cells 

 comparable to a dorsal lamina. In fig. 96 somewhat the same condition holds, though 

 the notochords are becoming reduced in size. This process of reduction goes on very 

 rapidly, till (in fig. 97) the notochordal grooves are of no more importance than the 

 dorsal grooves, and at the same time the ventral groove has become single and median. 



In fig. 98 the two dorsal grooves, and the ventral groove alone remain, and in 

 fig. 99 they are reduced to very small calibre and posterior to this they fuse and open 

 into the oesophagus. 



We thus see that the pharynx of Cephalodiscus has a pair of dorsal grooves 

 separated by a dorsal lamina, and a ventral hypo-pharyngeal groove, joined at the front 

 end by a peri-pharyngeal groove. At the point where the dorsal grooves join the peri- 

 pharyngeal groove, the sub-neural gland opens. The relation of the peri-pharyngeal 

 grooves to the ventral alimentary portion of the gut in Cephalodiscus seems to 

 corroborate the suggestion that the endostyle and thyroid are homologous with this 

 part of the pharynx in Balanoglossus. These grooves are all too small for water-currents 

 and their arrangement prompts an irresistible comparison with the mucus-grooves in the 

 pharynx of Urochorda, and although one can only cautiously apply close homologies 

 yet the already tentative comparison of the sub-neural gland (5 and 6) to that of the 

 Urochorda, is considerably strengthened by its similar relationship to the dorsal and 

 peri-pharyngeal grooves. 



In fig. 100 is seen a lateral view of the whole pharynx. The funnel-shaped mouth 

 (m) shows the three oral grooves (o.g.) on each side converging into the opening of the 

 pharynx. Round this opening is the peri-pharyngeal groove, running down from the 

 sub-neural gland (sn.gl.), from which also proceeds the dorsal groove. 



Down the dorso-lateral region runs the large paired pleurochordal ridge, which, 

 rising gradually from behind, bends over in front, and opens by the pharyngeal cleft 

 {p.c.) downwards and outwards to the exterior. 



From this description of the pharynx and its connected structures, it is not difficult 

 to understand to a great extent the functions performed by each part. 



The twelve branchial plumes with their pinnae are spread out from behind the 

 buccal shield, and currents of water and food-particles pass down the ciliated ventral 



