GENERAL STUDY OF TYPICAL CHORD ATES 19 



are continuous in some species. They consist of two lobes, and in the males 

 bear large stout claspers, deeply grooved along their posterior lateral margins. 

 The claspers are employed in mating with the females. 



3. Head. — The head like the trunk is greatly flattened, its margins con- 

 tinuous with the pectoral fins. It terminates in a pointed rostrum. It bears 

 dorsally a pair of large projecting eyes, without lids. Behind each eye is the 

 large spiracle or first gill slit, bearing a valve on its anterior face, marked by 

 parallel ridges which represent a rudimentary gill. On the ventral side of the 

 head is the mouth bounded by tooth-bearing jaws. The jaws and teeth are 

 commonly larger in the males than in the females. In front of the mouth are 

 the two nostrils, each provided with a fringed ear-shaped flap. Extending 

 posteriorly from each nostril to the angle of the mouth is a flap with a fringed 

 posterior margin. This is the nasofrontal process. On lifting up this process a 

 deep groove, the oronasal groove, will be found extending from the nostril into 

 the mouth cavity. This arrangement foreshadows the appearance of a closed 

 passage from the nostrils into the mouth such as is present in higher vertebrates. 

 Posterior to the mouth are five pairs of gill slits. In skates the pectoral fins have 

 grown forward above the gill slits and fused with the sides of the head, thus 

 shoving the gill slits to the ventral surface. 



4. Anus. — The anus or cloacal aperture is a large opening between the 

 bases of the pelvic fins. 



G. EXTERNAL ANATOMY OF A TELEOST 



For this purpose any common bony fish can be used, but the following 

 description is based upon the perch. The perch like other common fishes is a 

 member of the great order Teleostei. Obtain a specimen and note the following 

 points. 



1. Body and skin. — The body has the shape typical of aquatic animals, 

 thickest in the middle and tapering to each end. It is somewhat compressed 

 laterally. It is indistinctly divided into head, trunk, and tail. Trunk and tail 

 are clothed with scales arranged in diagonal rows, overlapping each other. 

 These scales are set in pockets in the deeper part of the skin (dermis), as may be 

 determined by removing one of them, and the superficial layer of the skin (epi- 

 dermis) forms a thin film over their surfaces. The head is covered by the soft 

 epidermis and in some regions bears small scales like those on the remainder of 

 the body. Beneath the scaleless portions of the epidermis of the head will be 

 noted large, thin, flat bones. These bones, which are the outer bones of the 

 skull, are in reality nothing but enlarged scales which have sunk from their 

 original superficial position to a deeper location. A lateral fine is present along 

 each side of the body. 



2. Head. — The head bears a terminal mouth bounded by well-developed 

 jaws. The terminal position of the mouth is probably more primitive than the 



