SUBPHYLUM VERTEBRATA 



This subphylum includes all chordates which possess: (1) a 

 segmented vertebral column, (2) a heart with at least two 

 compartments, and (3) a cranium. 



Since an extended study of a vertebrate, the frog, has already 

 been made, the object of the present study is that of affording 

 some direct contact with the lower or fish-like vertebrates. 



Class Elasmobranchii. The Spiny Dogfish, Squalus acanthiaa 

 (or Any Shark). — Sharks and rays constitute this class of fishlike 

 vertebrates. In a general way they resemble the true fishes but 

 differ from them so widely in structure that they are to be 

 regarded as constituting a separate class. 



This species is common along the Atlantic Coast of New 

 England. Preserved specimens retain their natural coloration 

 and body form to a remarkable degree. The adults are about 

 three feet long. Study a preserved specimen on a dissecting 

 tray. It should be thoroughly rinsed and kept moist during the 

 progress of study. 



Exterior. — The body form is typical of aquatic vertebrates, 

 being spindle shaped, with the deepest part between the first and 

 second thirds of the total length. This so-called "stream-line" 

 form is the most advantageous for progress in a liquid medium. 



The body is divided into head, trunk, and a portion caudad 

 of the cloacal aperture termed the tail. Such a morphological 

 tail is not found outside the chordate phylum. The skin is beset 

 with minute placoid scales each of which bears a tiny spine. The 

 resistance encountered in passing the finger from the tail toward 

 the head is due to the caudally projecting spines of these scales. 

 Embedded in the dermis is a bony plate from which the pointed 

 spine of dentine projects through the skin. The spine is covered 

 with enamel. View the surface of the skin with a lens. 



A light-colored line, the lateral line, extends along the sides of 

 the body from the head to the tail. Its location along the side 

 of the body suggested its name. In structure the lateral line is a 

 canal embedded in the epidermis. It communicates with the 

 exterior through pores. Within the lining of the canal there are 



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