been accompanied by a forward growth of the arms until finally they surround 

 the mouth and almost cover the head. A comparison of the structure and habits 

 of the various Cephalopods shows that the height of the visceral dome and power 

 of swimming are directly proportional. For example: Nautilus, a Tetrabranch, 

 the most primitive Cephalopod has a low visceral dome. It lives upon the 

 bottom and probably swims but little. The Octopoda, the lowest members of 

 the higher group , the Dibranchiata , have a higher visceral dome and , although 

 they frequent the bottom , are better swimmers than Nautilus. The Decapods , 

 the highest Cephalopods and the group to which the Squid belongs, have an 

 extremely elevated visceral dome and are free swimmers. In fact, the ability 

 to swim is directly dependent upon the elongation of the dorso-ventral axis; for 

 the Cephalopoda swim by ejecting water from the body cavity thru the siphon, 

 a tube formed by the posterior part of the foot, hence the longer the body the 

 less the resistance it encounters and the greater the capacity of the mantle cavity. 

 As the dorso-ventral axis becomes longer , i. e. as the visceral dome becomes 

 more elevated it becomes less vertical until , in the Decapods , it is horizontal. 

 Squid swim with equal facility either forward or backward , but because of the 

 above mentioned change of the dorso-ventral axis from the vertical to the 

 horizontal position, the ventral, not the anterior surface, is forward , the anterior 

 surface is uppermost , and the dorsal, not the posterior surface, is hindmost. The 

 fact that the axis which is usually vertical is horizontal in the Decapods has 

 naturally led to great confusion in terminology. The majority of systematists , 

 the English , French and some American morphologists have used the morphological 

 terms anterior , posterior , dorsal , and ventral in a positional or physiological sense 

 while the German zoologists , and some others have used the same terms in a 

 strictly morphological sense. In this description , the terms anterior , posterior , 

 dorsal , ventral are used in a morphological sense and the terms upper , lower , 

 fore , and hind are employed in their ordinary meaning. Hence the anterior 

 surface is uppermost , the ventral surface or point is forward , and the dorsal 

 point is backward. Further confusion of terms has arisen because of the partial 

 union of the head with the foot ; in this paper , the term foot denotes all the 

 structures formed from the primitive foot viz. the arms, tentacles, and siphon. 

 The mass formed by the fusion of the true or primitive head with a portion of 

 the foot , i. e. the arms , is called the head. 



The head of the squid is lance-shaped and is attached by a very short neck 

 to a cigar-shaped body so that the squid is nearly fusiform. The body consists 

 of a thick muscular mantle and a visceral mass enclosed by the mantle. The 

 otherwise cylindrical mantle tapers to a dorsal point. Ventrally the mantle ends 



