356 Vertebrata. 



lie in the body-wall on the side towards the atrium ; ova and 

 spermatozoa escape into this by the bursting of the organs, and are 

 ejected through the mouth. Segmentation is total, a blastula is 

 formed, the epiblast and hypoblast cells are little differentiated 

 (gastrula formation of the type depicted in Fig. 29) ; the embryo 

 hatches early, and the short larva swims by means of the cilia which 

 cover the surface. ' 



The class Leptocardii, which only includes the genus Amphioxus, 

 occupies in many respects the most primitive position among Vertebrata 

 (skeleton, nervous system, development, etc.), whilst in other respects it 

 is very peculiar and far from ancestral (atrium, etc.). 



The colourless Amphioxus lanceolatus, reaching about 7 c/m. long, 

 occurs on European coasts, buried in sand. 



Class 2. Pisces. [Fish.) 



The body is usually compressed and spindle-shaped ; head, body 

 and tail pass gradually into each other, the last is very muscular; 

 there is no neck, and movement of the head is usually very limited. 

 Many Fish differ more or less considerably from this general type; for 

 instance, some are so strongly compressed laterally that the animal 

 resembles a perpendicular plate ; in others, the head and body are 

 flattened dorso-ventrally ; in others again, the body is so elongated as 

 to be vermiform, or, on the contrary, it may be extraordinarily short 

 and bulky. Unpaired fins occur dorsally and ventrally upon the tail, 

 and on the dorsal side of the body. Usually two pairs of rather feeble, 

 flattened limbs are present ; sometimes the posterior, or even both 

 pairs, are wanting. It is characteristic of the Fish that the hind limbs, 

 the pelvic fins, have often moved far forwards ; close, or even anterior 

 to, the fore limbs or pectoral fins. 



The somewhat thin epidermis, as already mentioned, has no 

 stratum corneum ; goblet cells are often present, and their 

 secretion imparts to the skin its slimy character. The dermis often 

 contains ossifications, of which the best known are the so-called 

 scales, thin, bony plates lying in cavities of the dermis; they are 



Fig. 295. Diagrammatic 

 section of the skin of a Teleos- 

 tean, to show the scales. I 

 dermis, e epidermis, s scale.- 

 Orig-. 



often SO loosely connected with the latter, and lie so close to the surface,, 

 covered only by a thin connective tissue sheath and the epidermis, 

 that they are easily loosened by the movements of the animal and 

 fall off. They are usually imbricate, the overlapping edge being 

 posterior, and are then regularly arranged in rows. Cycloid 



