THE SKATE. 43 



66. The vent, or cloacal aperture, a large opening 

 with tumid lips, near the root of the tail. 



67. The abdominal pores (Figs. 18 and 19, pp. 55 and 

 57, a.p), small slits, one on each side of the middle line, 

 just posterior to the vent. 



68. A strong bar the ventral portion of the shoulder 

 girdle (Fig. 10, <rr) can be felt a little behind the last 

 pair of gill slits, and a similar bar the pelvic girdle (pu) 

 a little in front of the cloaca. Between these lies the 

 abdominal cavity, and in front of the shoulder girdle, 

 between the branchial apertures, is the position of the 

 pericardial cavity. 



69. The eyes, on the dorsal surface of the head, just 

 over the mouth : they are greatly depressed from above 

 downwards, and have no eyelids. 



70. The spiracles, two considerable apertures, one 

 behind each eye : they communicate with the cavity of the 

 mouth. The front wall of the spiracle is covered with fine 

 ridges, which constitute the pseudobranchia, the remains 

 of an embryonic gill. This is supported by a plate of 

 cartilage which may be easily felt, called the spiracular 

 cartilage (37). 



71. The sensory tubes, seen best on the ventral sur- 

 face, as delicate, transparent, winding lines, arranged sym- 

 metrically on opposite sides of the body, just beneath the 

 skin, on the surface of which they open by minute apertures. 



XII. Make a median longitudinal incision, first through 

 the skin, then through the underlying muscle, from 

 the posterior edge of the shoulder girdle to the 

 anterior edge of the hip girdle : make transverse 

 incisions from both ends of this, and reflect the 

 flaps of skin and muscle, so as to expose the 

 abdominal cavity. Make out the following : 



