154 MANUAL OF ANIMAL BIOLOGY 



derm, mesoderm, and endoderm present ; (6) the segmentation of 

 the body, which is internal in the Vertebrates ; (c) the presence of 

 a large coelom which, in the Vertebrates, consists of either a single 

 cavity or of an anterior and posterior cavity ; (d) the marked 

 bilateral symmetry ; (e) the still further development of the organs 

 and organ systems. In the later and more extended consideration 

 of the various phases of vertebrate organization, additional atten- 

 tion will be given to these features. 



We may now note the animals included in the main divisions of 

 the subphylum Vertebrata, the general characteristics of which we 

 have considered in sufficient detail. The classification of this 

 division of the animal kingdom does not offer the difficulties to 

 the systematist which are frequently encountered in the much 

 larger and more diversified Invertebrate groups, such as we have 

 noted, for example, in the Insecta. Seven classes of Vertebrate 

 animals are generally recognized, although some systematists 

 prefer to place all the fish-like types into one class, and thereby 

 reduce the total number of classes to five. 



Class I. Cyclostomata. This is a small class, but neverthe- 

 less contains a number of fish-like species which are interesting to 

 zoologists because of certain primitive characters which seem to 

 link them closely to Amphioxus. Thus (a) the notochord persists 

 throughout life; (6) a cartilaginous endoskeleton develops, is 

 functional, and never replaced by bone ; (c) the circular mouth 

 opening, with no jaws present, shows a striking resemblance to 

 that of Amphioxus ; (d) dorsal and caudal fins are present, but 

 paired fins are lacking. 



There are two subclasses recognized, namely, the Myxinoidea, 

 or Hagfishes; and the Petromyzontia, or Lampreys. The Hag- 

 fishes are marine and frequently deep sea. The Lampreys may be 

 found both in fresh and salt water. Several species of Lampreys 

 are known of which the most common is probably Petromyzon 

 marinus. The larval form of this species, known as Ammocoetes, 

 is of unusual interest in various ways, but particularly because it 

 resembles Amphioxus both in certain structural features and habits 

 even more closely than does a mature Petromyzon. After early 

 development it buries itself in the sand for a period of three or 

 four years, during which time it feeds on fine particles of food drawn 

 in through ciliary action, just as does Amphioxus. At the close of 

 this period it metamorphoses into the typical adult. (W. f. 68.) 



