



I 



PHYLUM CHORDATA 135 



1. EXAMPLE OF THE SUB-CLASS : THE DOG-FISH (Scyllium 

 canicula or Cliiloscyllium fuscuni). 



General external features. The general shape of the body 

 (Fig. 766) may be roughly described as fusiform ; at the anterior, 

 or head, end it is broader and depressed; posteriorly it tapers 

 gradually and is compressed from side' to side. The head termi- 

 nates anteriorly in a short blunt snout. The tail is narrow and 

 bent upwards towards the extremity. The colour is grey with 

 brown markings, or dark-brown above, lighter underneath. The 

 entire surface is covered closely with very minute hard placoid 

 scales or dermal teeth somewhat larger on the upper surface than 

 on the lower. These are pointed, with the points directed some- 

 what backwards, so that the surface appears rougher when the 

 hand is passed over it forwards than when it is passed in the 

 opposite direction. When examined closely each scale is found to 

 be a minute spine situated on a broader base. The spine consists 



FIG. 766. Dog-Fish (Chiloscyllium modestum). Lateral view. (After Giinther.) 



of dentine covered with a layer of enamel ; the base is composed of 

 bone, and the whole scale has thus the same essential structure as 

 a tooth. Along each side of the head and body runs a faint 

 depressed longitudinal line or slight narrow groove the lateral 

 line. 



As in Fishes in general, two sets of fins are to be recognised the 

 unpaired or median fins, and the paired or lateral These are all 

 Hap-like outgrowths, running vertically and longitudinally in the 

 case of the median fins, nearly horizontally in the case of the lateral : 

 they are flexible, but stiflftsh, particularly towards the base, owing 

 to the presence of a supporting framework of cartilage. Of the 

 median fins two the dorsal are situated, as the name indicates, 

 on the dorsal surface : they are of triangular shape ; the anterior, 

 , which is the larger, is situated at about the middle of the length of 

 the body, the other a little further back. The caudal fringes the 

 tail : it consists of a narrower dorsal portion and a broader ventral, 

 continuous with one another round the extremity of the tail, the 

 latter divided by a notch into a larger, anterior, and a smaller, 

 posterior lobe. The tail is heterocercal, i.e., the posterior extremity 

 of the spinal column is bent upwards and lies in the dorsal portion 

 of the caudal fin. The ventral or so-called anal fin is situated on 



