114 INVERTEBRATE ZOOLOGY 



The head and neck project from the large mantle cavity, into 

 which they can be partially withdrawn by means of powerful 

 retractor muscles, in very much the same way that a snail's 

 head and foot can be withdrawn into its shell. The siphon or 

 funnel, a large funnel-shaped organ at the base of the head, also 

 projects from it and can be similarly withdrawn. Gently probe 

 the mantle cavity and determine its extent. The mantle con- 

 stitutes the outer surface of the body. It will be seen to be a 

 cylindrical structure with thick, muscular walls, within which 

 lie 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 equivalents 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 dorso- 

 ventrally. 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 



