23O CLASSIFICATION OF VERTEBRATES. 



These same figures also illustrate another point. The rays of 

 the ventral side of the fin are supported upon the haemal arches 

 (Fig. 234, U) or upon elements (interspinalia) alternating with 

 these (Fig. 234, A~), while those of the dorsal side (and this 

 applies also to the dorsal and anal fins) are borne on similar. 

 interspinalia. 1 



The alimentary canal, though rarely straight (holocephali, 

 dipnoi, and some teleosts), does not present such convolutions as 

 are common in the higher groups. Teeth are almost univer- 

 sally present (except in sturgeon 2 and some lophobranchs), but 

 salivary glands never occur. The stomach is usually not 

 sharply marked off from the oesophagus, and it frequently has 

 a distinct siphon or U-shape. 



Gills are always present, the clefts, usually five in number 

 (six or seven in diplospondyli and some fossil forms ; four in 

 holocephali). Besides these the hyomandibular cleft persists 

 in most elasmobranchs and ganoids as a much smaller opening, 

 the spiracle, usually placed on the top of the head. In many 

 elasmobranchs, teleosts, and dipnoi the gill arches bear gill 

 rakers, small conical cartilages which extend into the clefts. 

 In the young elasmobranchs, sturgeon, and some teleosts the 

 gills extend outside the clefts as long fringes in the young. 

 Persistent external gills occur in Polypterus, and occasionally in 

 Protopterus, while they are retained for some time in other 

 forms. 



In the adult fish the brain occupies but a part of the cra- 

 nial cavity. It is characterized by the nearly complete disap- 

 pearance of flexures in the adult as viewed from the outside, 

 and by the slight development of the cerebral cortex, which 

 in the teleosts is lacking, being represented by a thin epi- 

 thelial layer, the cerebrum consisting in this group of only 

 the corpora striata of higher forms. The olfactory lobes are 

 distinct, and either lie close to the cerebrum (most teleosts) 

 or are removed some distance, in which case they are con- 



1 If the median or azygos fins have arisen by the coalescence of pairs of lateral folds 

 (p. 173), these interspinalia would then correspond to the basalia of the paired fins, as main- 

 tained by Dohrn and Mayer. 



2 The embryonic sturgeon has teeth. 



