CEPHALOCHORDA. 



351 



The buccal hood is chiefly noticeable for the great extent and thickness of its gelatinous 

 walls, a feature also of the whole fi-ont part of the animal. 



A section just behind the region, where the rostral fin runs into the apex of the buccal 

 hood, shews the great general development of the gelatinous tissue, when compared with the 

 area occupied by the muscles, &c. (Fig. 79). It will be noticed that in the section figured, 

 the tips of the buccal cirri have been cut through at this level. 



Fig. 77. 



Fig. 78. 



Fig. 77. Heteropleuron maldivense. Section through the middle of the preoral pit [P.P.). 



Fig. 78. Heteropleuron vialdivense. Section through the end of the preoral pit, showing the great development of 



the hood. 

 Fig. 79. Heteropleuron maldivense. Section through the anterior end of the head immediately behind the rostral 



fin. The buccal cirri are out through near their free ends. 



The side walls of the hood as they proceed backwards gradually become deeper, and at 

 their lower edges appears the ring of cartilage from which the buccal cirri spring. The hood 

 is of a broad shovel-shape, and the buccal ring thus has a V-shape with a pointed apex 

 and rounded sides. The cirri are 23 in number and are webbed half-way up their extent 

 except the last four on each side, which are small and free. Sections at different levels 

 in front of the velum shew the development of the hood (Figs. 78 — 80). In the front part of 

 the hood are a number of lymph spaces which traverse the gelatinous substance, and frequently 

 anastomose with one another. 



The gill slits in general structure are precisely similar to those of A. lanceolatus as are 

 also the branchial skeleton and endostyle. In one specimen a primary bar was found, possessing 

 two skeletal rods (Fig. 81), joined together at their base. They almost directly separated and 



