I04 PHYLUM MOLLUSCA 



structure with thick, muscular walls within which he all the viscera 

 of the animal ; its free edge is called the collar, as in the snail. It 

 is also necessary to observe that the mantle is not a paired organ, 

 as it is in the clam, but an unpaired one, as in the snail. The squid 

 has no foot, as has the clam or the snail, but morphological equiva- 

 lents of the foot are present in the arms and the siphon. 



Since in all mollusks the foot or its equivalent occupies a 

 ventral position, and the visceral mass a dorsal position, the 

 arms of the squid, together with the head, must be on its ventral 

 side, and the opposite end with the broad fins must be dorsal ; 

 the animal is thus enormously extended dorsoventrally. It will 

 be readily seen also that the mantle falls as a cylindrical fold from 

 the dorsal end about the entire body, exactly as it does in the case 

 of the snail. In fact, if the coils of the snail's visceral mass could 

 be straightened out, the mantle would fall as a cylindrical fold 

 from its dorsal end and terminate in the collar below, in the same 

 way as in the squid. The morphologically posterior side of the 

 animal is]that on which the siphon is situated ; the anterior side is 

 the opposite one. In common parlance, however, the head end 

 of the squid is called the forward end, and the fin-bearing end, 

 the hinder. The side bearing the fins is likewise called the upper 

 side or back, and the opposite side, on which is the siphon, the 

 under or lower side. These terms, although incorrect in a strictly 

 morphological sense, are much more convenient for general use 

 and will be employed hereafter in these directions. 



The mantle of the squid does not secrete an external shell, as 

 does that of the snail and the clam ; in a long pocket on the upper 

 side, however, is an elongate, horny structure, called the pen, 

 which is secreted by the mantle and is the equivalent of the shell 

 of other mollusks. 



Make a short, shallow incision in the upper surface of the man- 

 tle, beginning with the collar. Turn the flaps aside and note the 

 brown, horny pen lying beneath. Do not remove it at present, as 

 the dissection of the parts beneath might be disturbed by its 

 removal. 



Exercise 1. Make a drawing of the underside of the animal. 



