GENERAL ANATOMY OF THE SQUID. 335 



On the median dorsal line the mantle gives rise to a 

 short flap, which projects forwards over the head. 



d. Turn this flap over and slit the thin integument of 

 its inner surface, and notice inside it the anterior end of 

 the dark-brown, horny, internal shell, or pen. 



e. Make an incision through the integument, along the 

 median dorsal line, from the base of this flap to the pos- 

 terior end of the body. Turn back the integument on 

 each side of this incision, and notice the internal shell in 

 its capsule. 



( 1 . ) Raise up one end of the shell and pull it out of the 

 capsule, noticing that it is not attached to the walls in any 

 way, but is entirely free. 



(2.) The shell is thin, transparent, and horny, and con- 

 sists (a) of a central shaft, which runs from end to end, 

 like the quill of a feather, and which is strengthened by 

 three parallel ridges, and (6) of two lateral portions, like 

 the vanes of a feather, one on each side of the posterior 

 live-sixths of the shaft, and strengthened by a marginal 

 ridge. 



(3.) Make a sketch of the shell. 



(4.) Notice that the capsule of the shell is a closed sac, 

 lined by a delicate membrane, and without communica- 

 tion either with the exterior or with the body cavity. 



f. In a dorsal view of the head, notice the protruding 

 eyes, and three pairs of arms (Fig. 176, a, a', a"), which 

 are visible in a dorsal view. Notice that these arms are 

 symmetrically arranged with reference to the dorsal me- 

 dian line. 



g. Make a sketch of the dorsal view of the animal. 

 3 . In a ventral view notice : 



a. The delicate parallel bands of muscles which extend 

 from the body to the lateral edges of the fin. 



