CHAPTER IV 



SCYLLIUM, A CHORD ATE WITH JAWS, STOMACH, AND FINS 



Externals. — The dogfish possesses an elongated body with a 

 distinct head and tail, the latter provided with a tail-fin of which 

 the ventral lobe is larger than the dorsal (heterocercal). There 

 are two median dorsal fins. The most obvious advance over 

 the Cyclostome condition is the possession of paired fins, of 

 which there are two pairs : a pectoral and a pelvic. 



The head has paired nasal sacs and eyes. The mouth 

 is situated some distance behind the anterior end of the snout. 

 Behind the mouth on each side are six openings into the 

 pharynx. The first pair of these is small and more dorsally 

 situated than the others ; it is the spiracle. The remaining 

 five are the gill-slits, numbered i to 5. The anus lies in a 

 cloaca (joint opening of the alimentary and urino-genital 

 systems) in the midventral line behind the pelvic fins, and on 

 each side of it is a small pore (the so-called abdominal pore) 

 communicating with the coelom. In the male, there is a pair 

 of claspers on each side of the cloaca. 



Denticles. — The body is covered all over with small sharp 

 spikes, with the points directed backwards. These are the 

 placoid scales or denticles. They are made of dentine covered 

 over with a cap of enamel. Dentine is a hard substance pro- 

 duced by mesodermal cells beneath the epidermis ; and it is 

 identical with the substance of which the teeth of all verte- 

 brates are made. It consists of a calcified ground-substance 

 in which filamentous processes of cells are to be found, but no 

 cells themselves. In this particular it differs from bone. The 

 enamel is formed from the ectoderm. On the inner rim of the 

 jaws just inside the mouth, denticles are also found. They 



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