CHAPTER VIII 

 VERTEBRATE ANIMALS SCYLLIUM CANICULA 



Introduction to Craniata External features, Scyllium Integument 

 Muscular system Endoskeleton. 



A Fish Scyllium canicula, the Dogfish. 



The next type to be studied is that of a fish, and the species 

 taken is that known as the lesser spotted dogfish, and to the 

 Zoologist as Scyllium canicula. It is practically a little shark, and 

 is the smaller of two dogfish caught commonly around the British 

 coasts. The larger, more rare 5. catulus, the greater spotted dog- 

 fish or rough hound, is very similar, so much so indeed that young 

 specimens of it are sometimes mixed in with the smaller species 

 supplied to the laboratory, and the differences between them are so 

 slight as to be negligible for our present purposes. 



Compared with the earthworm the dogfish represents a great 

 advance in the animal series, for it is a vertebrate animal belonging 

 to the phylum Chordata, the same phylum as the frog, with which we 

 commenced our studies, and so possessing many features in common 

 with that animal. The Chordata are often divided into two groups : 

 the ACRANIA, comprising a number of less familiar forms such as 

 the lancelet Amphioxus and sea squirt Ascidia, etc., whose relation 

 to the other members of the phylum are not clear, and all the remain- 

 ing forms, the Craniata, marked off by the possession of a head and a 

 vertebral column. These are the Vertebrata in the strict meaning 

 of the term, and contain the following classes : PISCES, the Fish ; 

 AMPHIBIA, the Frogs, Newts, etc. ; REPTILIA, the Reptiles ; 

 AVES, the Birds ; and MAMMALIA, the Mammals. The dogfish 

 is important because it is a representative of the Elasmobranch 

 Fishes, one of the lowest groups, including sharks and rays, whose 

 skeleton remains cartilaginous throughout life. This group, although 

 specialised in some respects, retains a number of primitive features, 

 and is consequently of considerable interest to the comparative 

 anatomist. The primitive nature of the group is further demon- 

 strated by the fact that the earliest known remains of fossil verte- 

 brates belong to it. 



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