MORPHOLOGY OF THE VERTEBRATA. 



D O G-F I S H. 



Class — Pisces. Order — Elasmobranchii. Sub-order — Selachoidei. 

 Type— Scylium (Dog-Fish). 



Note elongated pointed snout, and that mouth is on under 

 surface of the head, some distance from anterior extremity of 

 body. It is of great capacity, being the common entry to both 

 the digestive and respiratory organs.; and as the transmission of 

 the food to the stomach, and of respiratory currents to the gills, 

 is performed by similar acts of deglutition, the bony arches which 

 encircle the mouth are large, and there is a complicated mechanism 

 for directing the nutriment and the oxygenating water each to 

 their respective destination. In front of the mouth are the two 

 nasal apertures, and on either side of it the external auditory 

 openings. Following at some distance from the mouth are five 

 vertical clefts on each side — these are the external openings of 

 the branchial sacs, which look backwards. Immediately behind 

 the last branchial cleft are the pectoral fins. In front of the 

 junction of middle third with posterior third of body are the 

 ventral fins, with the anus between their apposed bases. The 

 attenuated posterior end of body is continued into the heterocercal 

 tail. On upper surface of head behind each eye is an opening— the 

 spiracle — which leads into the cavity of the mouth, and is the 

 remains of the first branchial cleft of the embryo. On dorsal 

 median line are two fins — the dorsal -fins— one being anterior and 

 the other posterior. In some species of dog-fish a sharp thick 

 spine is inserted in front of the dorsal fins, which is known as 

 a dermal defence, this being of the same nature as the fossils 

 ichthyodorulites. The entire body is covered over by sharp, 

 closely set spines, arranged in regular oblique rows — the whole 

 constituting the shagreen. Each spine or denticle l agrees with a 

 tooth in structure, being developed in a papilla of the true skin, 

 or corium. A wavy line, the lateral line, runs on each side of the 

 body from behind the head to the tail. As the description of this 

 is more easily seen in the cod ; its structure will be described a 

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