EXTERNAL CHARACTERS 



The spindle-shaped body tapers from near the middle toward 

 both head and tail; the head is flattened on both the dorsal and 

 ventral sides, while the remainder of the body is nearly round, with 

 a lateral compression which is not pronounced except in the caudal 

 portion. 



The general color of the back and sides is gray; darkest above, 

 where the skin is spotted with scattered, small, round, light spots. 

 The color of the upper parts shades into the yellow white of the 

 ventral surface. 



Can head, trunk, and tail regions be distinguished? If so, what 

 characters determine the extent of each? 



A little above the middle of the side of the body is the lateral 

 line, (distinguished partly by color, partly by being slightly ele- 

 vated), which extends from the back of the head to the tail. Cut 

 through the skin across the lateral line at several points along the 

 body and notice the canal which lies in the dermis under the lateral 

 line. This is the lateral line canal, which opens to the surface by 

 numerous pores (too minute to be seen), and contains a series of 

 special sense organs along its dorsal and inner surfaces. Near the 

 base of the caudal fin the lateral line canal passes into a groove 

 which continues the lateral line to within a short distance of the 

 edge of the fin. The lateral line canal in its development begins 

 as a groove along the side of the body which becomes closed by the 

 fusion of its edge except in this terminal portion. 



The open groove does not appear in Eugaleus. 



In the midline of the body are two triangular dorsal fins, each 

 attached to the body for about half its length. The basal portion 

 of each is thick and muscular, and contains supporting cartilages 

 embedded in the muscles. The remainder of the fin is flexible and 

 semi-transparent, horny finrays being faintly visible between the 

 layers of skin. In front of each fin is a strong spine which seems 

 to serve both as a cutwater and a weapon of defense. (Eugaleus 

 has no spines.) 



The broad, paired, pectoral fins, having the general characteris- 

 tics of the dorsals, spring from the ventral edges of the body just 

 back of the head. A hard bar of cartilage connecting the bases of 

 the pectoral fins can be felt through the skin of the ventral surface 

 of the body. This is the ventral part of the pectoral girdle. 



Farther back, and also on the ventral surface, are the paired 

 pelvic fins. The pelvic girdle can be felt through the skin between 

 the bases of these fins. If the specimen is a male, it will have a 

 fingerlike process projecting backward from the base and along the 

 inner side of each pelvic fin. These organs, which attain a consid- 

 erable size in adults, are modified portions of the pelvic fin used as 

 copulatory organs. They are named variously claspers, myxop- 

 terygia, or pterygopodia. A groove runs along the dorsal side of 

 the clasper from the tip to near the base, where it opens into a long 

 sac (glandula pterygopodia) extending some distance in front of 

 the pelvic fins just within the skin of the ventral surface. By feeling 

 a cartilaginous axis of the clasper can be discovered, which extends 

 to the tip of the organ. On the dorsal surface and near the end of 

 the clasper is a sharp grooved spine on the outer side, and a strongly 

 recurved hook on the inner side, both almost hidden by a flap pro- 

 jecting from the inner edge of the groove. This flap is stiffened by 

 a series of small cartilages similar to the radial cartilages in the 

 bases of the fins. 



