18 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



of trunk and tail, the pelvic fins. In male dogfishes, the medial side of each pelvic 

 fin is modified into a stout process directed posteriorly. These processes are 

 called claspers and are used by 'the males in mating with the females. All of 

 the fins are supported by slender flexible rays, the dermal fin rays, imbedded in 

 the skin of the fins. 



5. Anus. Between the two pelvic fins is a large opening, the anus or cloacal 

 aperture. This is the opening of a chamber, the cloaca, which receives the end 

 of the intestine and the terminations of the ducts of the kidneys and reproduc- 

 tive organs. The anus of the dogfish and of the majority of vertebrates does not 

 quite correspond to the anus of man and other mammals, which receives the 

 intestine only, but the term is commonly used to designate both classes of 

 opening. 



F. EXTERNAL ANATOMY OF THE SKATE 



The skates or rays are, like the dogfish, elasmobranch fishes but highly 

 modified as regards external form and proportions. Obtain a specimen and 

 examine. 



1. Body and skin. The body is divided into a greatly flattened anterior 

 portion comprising head and trunk and a slender posterior portion, the tail. 

 The broad, flat form is characteristic of bottom-feeding fishes (see N, pp. 97-99) 

 and results from a shortening of the dorsoventral axis and an elongation of the 

 transverse axis. The tough skin contains scattered scales of which the spines 

 project conspicuously. These scales are of the same type already noted in the 

 dogfish, consisting of a basal plate imbedded in the skin and a projecting spine, 

 directed posteriorly. The scales of the skate are much larger and fewer in number 

 than those of the dogfish. They are definitely arranged in lines and groups, the 

 arrangement differing somewhat in the two sexes. In females there are scales 

 over the lateral expansions of the trunk and several rows of scales on the median 

 dorsal part of the trunk and dorsal surface of the tail, while in males the greater 

 part of the lateral expansions is devoid of scales, there are fewer rows along the 

 middle of the back and tail, and the scales on the margins of the head are enlarged. 

 In males, furthermore, there are two rows of curious erectile spines on the lateral 

 expansions of the trunk about an inch in from the margin; these can be erected 

 and lowered into depressions in the skin. Are there any scales on the ventral 

 surface? Note the marked differences in color between the dorsal and ventral 

 surfaces. 



2. Fins. Like the dogfish the skate is provided with median and paired 

 fins, but the former are much reduced. They consist of two small dorsal fins 

 on the dorsal side of the end of the tail. The pectoral fins are enormously 

 enlarged, forming the lateral expansions of the trunk already mentioned several 

 times; they are confluent anteriorly with the margins of the head. The pelvic 

 fin? are smaller and immediately posterior to the pectoral fins with which they 



