THE SKATE. Raia hatis or Raia clavata. 



Examine the living skate in an aquarium, and note that it swims in an undulating 

 manner principally by the action of the large pectoral fins, that in breathing the 

 water enters by the mouth and the two spiracular apertures, and after passing over 

 the gills, escapes through the gill slits, and also by the spiracular apertures ; note 

 also that in breathing the spiracles and gills beat rhythmically from 30 to 32 times 

 per minute, and that the mouth sometimes opens and closes at the same rate. 



A. External characters — 



1. Examine the skate, noting — 



(a) The rhomboidal form, and that it is depressed from above downwards. 



(6) The colour — dark dorsally and ventrally in R. hatis, dark dorsally, white 



ventrally in R. clavata. 

 (c) The thick skin — smooth in R. hatis, rough in R. clavata, owing to the 



presence of small denticles (placoid scales). 

 {d) The large dermal denticles scattered more or less over the dorsal and ventral 



surfaces of R. clavata, but only along the dorsal surface of tail of R. hatis. 

 (e) The delicate sensory tubes beneath the skin, and their numerous external 



apertures (best seen in R. hatis). 

 (/) The anterior projecting rostrum. 

 (g) The large triangular-shaped pectoral fins. 

 (h) The smaller bi-lobed pelvic fins. 

 (i) The long narrow tail with its terminal caudal fin. 

 (j) The pair of elongated claspers (in the male only). 



2. Make a sketch of the dorsal and ventral aspects, indicating as far as possible 



the above characters ; and also — 



(1) In the dorsal view — 



(a) The oval-shaped depressed eyes. 



(b) The spiracular apertures, spiracular cartilages, and pseudo-branchiae. 



(c) The minute apertures of the aqueducti vestibuli at the posterior end 



of the head. 



(d) The pair of dorsal fins. 



(2) In the ventral view — 



(a) The large mouth with its upper and lower jaws covered with flat 



enamelled teeth (generally raised into sharp ridges in male of R. 

 clavata). 



(b) The nasal apertures, connected by the naso-buccal grooves with the 



mouth. 



(c) The fronto-nasal process — the fold of integument between the naso- 



buccal grooves. 



(d) The ten gill clefts arranged in two semi-circular rows, and behind 



the last gill openings a pair of slit-like markings resembling a sixth 

 pair of clefts. 



(e) The ridges formed by the pectoral and pelvic girdles. 

 (/) The large cloacal aperture. 



(jff) The pair of small abdominal pores posterior to it. 



3. Cut out and sketch one of the large denticles, noticing that it consists of a 



flattened bony base buried in the integument and a projecting sharp 

 enamelled spine. 



