TYPE MOLLUSC A. 313 



with the exterior. This couditiou seems to be a secondary 

 modification of one in which the oviduct and vas deferens 

 open independently at widely separated points a condition 

 which is represented by a few Opisthobrauchs. 



Suborder Tectibranchia. 



The Tectibrauchiates are those Opisthobrauchs which 

 present the smallest amount of modification from what has 

 been considered the typical Gasteropod structure. A more 

 or less developed mantle-fold is usually present, sometimes 

 sufficiently voluminous to cover in the single branchia which 

 persists (Bulla), but frequently represented only by a slight 

 fold, which leaves the branchia exposed (Aplysia, Gasterop- 

 terori). A shell is very generally present, sometimes well de- 

 veloped (Bullet), but in other cases reduced to a plate-like 

 structure enclosed within the mantle which has been reflected 

 over it and the lips of the reflected portion meeting and fusing 

 (Aplysia, Pleurobranchus). The visceral hump, however, is as 

 a rule low and elongated in the direction of the long axis of 

 the body instead of at right angles to it, as in the majority of 

 Prosobrauchs. In many members of the group the foot pos- 

 sesses a broad creeping surface, but its margins are prolonged 

 into broad thin wings, the parapodia, which may be bent up- 

 wards, as in Aplysia, so as almost to enclose the body. 



The Tectibranchiates are divisible into two groups accord- 

 ing to their habits, in accordance with which the form of the 

 foot and especially of the parapodia is modified. Those forms 

 which possess a broad flat sole to the foot have a creeping 

 habit ; but there are many forms which are pelagic in habit 

 and present many modifications of structure in adaptation to 

 this mode of life, and were consequently classified at one time 

 as a distinct order, the PTEROPODA, and consequently call for 

 special mention. One of the most characteristic features of 

 this group is the foot, which is limited to the anterior portion 

 of the body and consists of a small median portion and two 

 lateral wing-like parapodia arising from the sides of the 

 median portion, and by the rapid flapping of wdiich the 



