78 A. E. Yerrill — The Opisthoteuthidce. 



indicate the position of the posterior pole, though many 

 writers consider it as dorsal, being influenced by the position of 

 the anus. The latter is formed later than the mouth and 

 shell-gland and is too variable in position and mode of origin 

 to be of much significance in this connection. Its normal and 

 usual position in the larvae is ventral, posterior to the mouth. 



The theory adopted by Huxley, Lancaster, and many 

 others,* that the actual dorsal surface of a cephalopod is the 

 anterior region ; that -the actual posterior end of the body is 

 the dorsal side ; and that the actual ventral side is the pos- 

 terior surface, I consider as entirely erroneous and without 

 any real foundation. One of the fundamental errors that has 

 led to this theory is the wrong interpretation of the gastropod 

 body. Thus Lancaster took the specialized flattened form, 

 seen in limpets and chitons, as the normal or primitive form, 

 and therefore made the long axis of the foot the antero-pos- 

 terior axis of the body. When discussing the more common 

 and more normal forms, with a body prominent above the foot, 

 whether spiral or not, he called the actual body the " visceral 

 hump " and treats it as a sort of hump, or dorsal appendage of 

 the body. Hence, should it stand nearly at right angles to the 

 foot, its front surface would become anterior and its back or 

 ventral surface would become posterior. The same erroneous 

 reasoning would naturally compel the ordinary cephalopod 

 (fig. 6) to stand on its head (the arms or pedal organs being 

 placed around the mouth), and in that position the body would 

 correspond to the " visceral hump or dome " of the gastropods. 

 In view of all that is now known of the embryology, anatomy, 

 and homologies of the mollusca, such theories seem to me 

 quite untenable. To me the more natural and correct view 

 appears to be to consider the foot as a ventral appendage, and 

 the " visceral dome " of a gastropod as the true hody, whether 

 it be high or low, flat or round, straight or twisted, and regard- 

 less of the position of the anal pore.f 



That this is the true view can be easily shown by the study 

 of the development of the larva in the proveligerj and veliger 

 stages, for the larva early acquires definite anterior and pos- 

 terior ends, and dorsal and ventral surfaces are diiferentiated 



* This view is adopted and discussed at some length in the recent Text-Book 

 of Comparative Anatomy, by Dr. Arnold Lang, 1896. 



f The anus is variously situated in gastropods that have the shell small or 

 abortive, though the normal position is at one side of the neck-region in the gill- 

 cavity, when a spiral shell is present, because the gill-cavity must necessarily be 

 anterior in such cases. Huxley regarded the mouth and anus as representing 

 respectively the " true morphological " anterior and posterior poles of the body. 

 This view when applied to the various positions of the anus in Opistho- 

 branchs, leads to very strange results. 



X The stage that I call proveliger is that which precedes the formation of the 

 functional anal pore and complete intestine, but in which the shell-gland is pres- 

 ent.* The subveliger is the next stage ; in this the true shell begins but the intes- 

 tine is still incomplete. 



