ii8 



ELEMENTS OF BIOLOGY 



AMPHIBIA 



limbs either functional or rudimentary (except in the class Cyclos- 

 tomata), and reproduce solely sexually. The ccelom is separated 

 into two or three compartments. There are seven classes: 



Class L CYCLOSTOMATA 



(Fig. 66). Lamprey Eels and 

 Hagfishes. Do not have 

 paired limbs. The mouth is 

 at the extreme anterior end 

 and has no jaws. The em- 

 bryos of these forms in many 

 respects resemble the adult 

 Cephalochordata, Amphi- 

 oxus. Found in both fresh 

 and salt water. Lampreys are 

 used for food in some coun- 

 tries. 



Class IL ELASMOBRANCHII 



(Fig. 67). Sharks and Rays. 

 The internal skeleton is en- 

 tirely of cartilage and the in- 

 tegument is covered with 

 SCALES consisting of a bony 

 basal plate bearing an enam- 

 eled spine. The gill openings 

 are not covered. 



Class in. PISCES (Fig. 68). 

 The true Fishes. The skele- 

 ton always contains more or 



FIG. 69. FROG 

 Fig. 69. — A common frog, Rana pipiens. 

 As in all modern Amphibia, the integu- 

 ment is free from scales. Compare this 

 feature with the well-developed scales 

 of fishes (Fig. 68) and with Reptilia 

 (Fig. 70). Note also the absence of claws; 

 compare with the reptile (Fig. 70). 



less bone. The gills are cov- 



ered by a bony shield. Scales 

 are not always present; when 

 present they consist of smooth plates without the enamel that char- 

 acterizes the scales of the Elasmobranchii. Practically all the forms 

 that are commonly known as fishes belong in this class, although 



