2 



A SURVEY OF THE MATERIALS 



It is useful, before beginning the study of vertebrates, to 

 make a preliminary survey of the kinds of animals making 

 up this group. There must be some order of arrangement of 

 these organisms, some scheme of classification associating 

 similar species together and separating these by means of a 

 graded series of hierarchies which classify species from the 

 similar to the dissimilar. A classification thus reflects observ- 

 able morphological differences and is presumed to reflect, at 

 least partially, the evolutionary history or phylogeny of the 

 entire group. 



The preliminary grouping of organisms in Table 2-1 

 should not be viewed as a formal classification but rather as 

 an introduction to some of the views regarding the natural 

 grouping of species and the interrelationships of such groups. 

 These views are to be studied in the following chapters in 

 more detail and evaluated in terms of the morphological 

 facts. No conclusions, as expressed by a formal classification, 

 can be reached until all possible information has been accu- 

 mulated and reviewed, and even then the student may wish 

 to reserve judgment. Discussing anatomical or developmen- 

 tal information within a preconceived taxonomic system 

 robs it of much of its meaning. Working toward an under- 

 standing of taxonomy through a study of the many kinds of 

 vertebrates adds a new dimension. 



The preliminary system of Table 2-1 is, therefore, not to 

 be memorized nor treated as a formal classification by 

 equating its major divisions with the classes, orders, or fami- 

 lies, of other systems. It is intended only as a reference 

 framework, a starting point for a study of the phylogeny of 

 the vertebrates as revealed by their morphology. The names 

 in the right-hand column are, with a few exceptions, genera. 

 Those preceded by an asterisk are of fossil forms. The names 

 in the other columns are of increasingly more inclusive cate- 

 gories from right to left. 



LIVING REPRESENTATIVE VERTEBRATES 



A description of the more important external features of 

 a few of the animals in this list will reveal some of the di- 



The Vertebrates and their 

 External Anatomy 



versity of the vertebrates and serve as an introduction to the 

 terminology of comparative morphology. Animals with 

 which the student is already familiar (e.g. the rat) are noted 

 only briefly, whereas more detailed descriptions are used to 

 supplement the illustrations of forms likely to be new to him. 

 No attempt will be made at this time to discuss the inter- 

 relations of these vertebrates since such features as agnath 

 ("jawless") versus gnathostome ("jawed mouth") are more 

 properly discussed under the anatomical structure on which 

 the separation is based. By careful attention to the details 

 of the external anatomy of the following animals, along with 

 comparisons where this is possible, an introductory famili- 

 arity with some of the generic and larger group names will 

 be achieved. These names will figure importantly in the fol- 

 lowing chapters and in the laboratory work. 



Cyclostomes 



The two kinds of living agnath fishes are grouped together 

 as the cyclostomes or "round mouths." The lamprey eels 

 (Figure 2-1), which include the genera Pelromyzon, Lamp- 

 etra, and Entosphenus, are fairly large fish with a smooth 

 skin. Anteriorly there is a funnel-shaped mouth (buccal fun- 

 nel) armed with circlets and rows of cornified or horny 

 teeth, as is also the "tongue" or rasping organ. The "tongue" 

 is moved back and forth to rasp a hole in the prey animal. 

 The eyes are high on the sides of the head and are covered 

 with a whitish skin, the cornea (in preserved specimens). 

 There are small bumps on the head, mainly in front of the 

 eye, which indicate the positions of the sensory organs of 

 the lateral-line system. 



Behind the eye is a row of seven branchial (gill) pouch 

 openings, each having a small papilla and leaf-like valves. 

 There are two dorsal fins far back on the body. The poste- 

 rior dorsal fin is continuous with the caudal fin, which 

 extends around the posterior end of the body and forward 

 to the cloacal aperture, through which extends the urinary 

 papilla. In the female a small anal fin is present just behind 

 the cloaca. There are no lateral fins. 



The hagfishes {Myxine and Eptatretus) are much like the 

 lampreys in their general shape. The nasohypophyseal duct 

 opening, seen high on the top of the head in the lamprey 



18 



