36 A MANUAL OF ZOOLOGY 



Upon being irritated he quickly projects black ink, 

 which is secreted in the ink sac, through the siphon and 

 he is suddenly enveloped in a cloud of black fluid which 

 affords him an easy means of escape from his enemy. 



His large eyes, his great variety of motions of tenta- 

 cles and bo(Jy, and his change of color, give him on the 

 whole a yery_x>dd. appearance. 



PHYLUM CHORDATA 



ELASMOBRANCHn 

 The Dog-fish (Musteius canis) 

 EXTERNAL FORM— What is the general fofm? Boes 

 it belong to the fishes? Why? (See Hetwig, pag'e 557). 

 Can you ' determine divisions: head, bb(4y,'aiid tail? 

 Note the rostrum or beak, anterior to the 'mouth 

 and eyes. Note the position and shape ot the eyes. 

 Are there eyelids? With the teilaculum hpok up the 

 membranous fold under the eye, 'and di'aw'it up 'over the 

 eye. This is the nictitating membrane. ' Has it an ana- 

 logue among the higher types? Immediately posterior 

 to the eye note a small opening, the spfracie.' Posterioi; 

 and somiewh'at vfentral to the spiracle" you will' see five 

 slits which are the gill clefts. Is there an arrangement by 

 which they may be closed? Lift up the flap of skin cover- 

 ing them and notice the gills "or branchial appa:ratus. Qan 

 you demonstrate .tie connectibfa of the gill-clefts and the 

 spiracles with 'thfe pharynx, and 'mouth? What Would you 

 Infer as to the f unction ^6f the spiracles from 'the deriva- 

 tion of the -^ord? What reaisons would you glVe that 

 water goes froto the moiith through the sijiracles ani the 

 gill-clef td instead of in the opposite- direction? On the 

 ventral side of the head, some distance from the front 

 of the beak, note the triangular-shaped mouth. Are the 

 teeth placed in rows? Lbok at them with the hand lens. 

 How would , the fish seize food thrown upon' the surface 

 of the water above it? Open the mouth and examine the 

 tongue. Is there any connection between the size, length, 

 and shape, and the absence of molar teeth as found in the 

 higher forms? Can ydii assign any reason for the flat 

 Burface of the underside of the head and the rostrum? 



