416 



Comparative Morphology of Chordates 



by bony armor which was especially strongly developed over the head. 

 Little is known concerning the internal skeleton. It seems probable 

 that they possessed the notochord, although they lacked any internal 

 skeletal parts capable of becoming fossilized. They were small animals, 

 usually less than a foot long. 



The Ostracodermi comprised a considerable number of members 

 which were highly diversified as to form of body and as to the nature 

 and distribution of the external bony plates. It is probable that the 

 ostracoderms and cyclostomes are remotely related. They are included 

 together in the group Agnatha. 



II. Class Pisces 



Fishes are now the dominant aquatic animals of the world. In 

 number of species they are exceeded, among vertebrates, only by the 

 birds. Their extremely diverse anatomic adaptations enable them to 

 live in fresh, brackish, or salt water and, in fact, to inhabit successfully 



a variety of aquatic environments 

 ranging from clean, well-aerated water 

 to mud. Some live near the surface. 

 Others, living at depths beyond the 

 penetration of sunlight, are equipped 

 with highly specialized integumentary 

 light-producing organs. In size they 

 range from the whale shark, which 

 may attain a length of 45 feet, to the 

 Philippine pygmy (Pandaka pygmaea) 

 which, when full grown, is about two 

 fifths of an inch long. 



In addition to their permanent 

 gills and paired fins, fishes have the 

 typical aquatic skin — that is, a skin 

 richly provided with mucus-producing 

 glands, usually of the simple unicel- 

 lular type (Fig. 321). In most fishes, 

 too, the skin produces scales which 

 exist in great variety of form and structure but are alike in being 

 composed of calcareous material. Taking the group as a whole, the 

 endoskeletal materials include the notochord, cartilage, and bone. 

 There is always a more or less well-developed vertebral column except 

 in the sturgeons and the lungfishes, in which the notochord persists 

 and vertebrae are represented only by segmentally arranged and dis- 

 continuous neural arches surrounding the spinal cord and, in the 

 caudal region, hemal arches enclosing the main caudal blood-vessels 



Fig. 321. Skin of lungfish, Pro- 

 topterus; section perpendicular to 

 surface; much enlarged, (c) Dermis 

 (corium); (e) epidermis; (g) multi- 

 cellular gland; (u) unicellular 

 gland. (Courtesy, kingsley: "Com- 

 parative Anatomy of Verte- 

 brates," Philadelphia, The Blakis- 

 ton Company.) 



