XIII 



PHYLUM CHORDATA 



243 



There are no branchial rays, but the branchial arches bear a series 

 of gill-rakers with cartilaginous supports. 



The pectoral arch (Fig. 915, pel.} is a stout cartilage with two 

 pairs of investing bones, the clavicles on the coracoid, and the 

 cleithm on the scapular regions. The latter are connected with 

 the skull by post-temporals. The skeleton of the pectoral fin 

 consists of a stout basal cartilage (has.), an elongated, tapering, 

 central axis made up of a number of short cartilaginous segments, 

 and two rows of jointed cartilaginous rays extending out on either 

 side of the axis so as to support the middle part of the expanse of 

 the fin. The pelvic arch is a single cartilage, produced forwards 

 into an elongated rod-like epipubic process (Fig. 918). The skeleton 



of the pelvic fin is similar to that 

 of the pectoral. 



Digestive Organs. The teeth 

 (Fig. 917) are of a remarkable 

 and characteristic shape. There 

 are two pairs of large compound 



FIG. 918. Ceratodus forsteri. Pelvic arch and skeleton of pelvic fin. (After Giinther.) 



teeth of similar character, one pair (the palatine, d) on the roof 

 of the mouth (palatopterygoid bone) and the other (splenial) 

 on the lower jaw. Each is a curved plate with the convex border, 

 which is directed inwards and somewhat backwards, entire ; while 

 the concave border presents a series of six or seven vertical, ridge- 

 like projections or cusps. In addition to these, there are, in front 

 of the palatine pair, a pair of much smaller, simple, somewhat 

 chisel-like vomerine teeth (d'} placed close together and directed 

 vertically. In the embryo each tooth is represented by a number 

 of separate denticles which subsequently coalesce. 



In the enteric canal the chief feature of special interest is the 

 presence, throughout the length of the intestine, of a spiral valve 

 similar to that of the Elasmobranchs and Ganoids. The rectum 

 opens into a small cloaca. A pair of abdominal pores open just 

 behind this. 



Organs of Respiration. Ceratodus combines aquatic respira- 

 tion by means of gills similar to those of ordinary fishes, with 

 aerial respiration by means of a lung. 



There are four pairs of gills, each consisting of a double row of 

 gill-filaments supported on the branchial arches. A rudimentary 



