Elements of biology, with special Elements of biology, with special reference to their rôle in the lives of animals elementsofbiolog00buch Year: 1933 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, A
Elements of biology, with special Elements of biology, with special reference to their rôle in the lives of animals elementsofbiolog00buch Year: 1933 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
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