ORDER OPISTHOBRANCHIATA. 



Suborder TECTIBRANCHIATA. 



Hermaphrodite, Opisthobranchiate, Gastropods, with one branch- 

 ial plume situated on the right side ; mantle and shell developed. 



This suborder differs from the Nudibranchiata in the asymmetrical 

 unpaired gill, the development of a mantle and shell, etc. It is a 

 synthetic type, from which in the remote past, the Nudibranchiata 

 and Pulmonata have no doubt been derived. 



Fischer has proposed a classification of this group which seems to 

 be the best yet published covering the entire suborder. In the fol- 

 lowing pages his general arrangement is followed ; but with numer- 

 ous minor modifications. The group is primarily divided into three 

 sections, whose characteristic features are well expressed in their 

 names : 



I. CEPHALASPIDEA : Head with a fleshy disc or shield. (Bulla, 

 etc.). 



II. ANASPIDEA : No head-disc nor dorsal shield. (Aplysia, etc.) 



III. NOTASPIDEA : no head disc ; back protected by a large 

 shield or notseum, and by a true mantle and shell. ( Umbrella, etc.) 



The first of these sections will now be considered. 



I. TECTIBRANCHIATA CEPHALASPIDEA. 



" All of these animals are characterized by the presence of a head- 

 disc, distinct from the back, bearing the sessile eyes and the tentacles 

 when present. This disc appears to be a tactile organ. It varies 

 in many ways, furnishing good characters for classification. Some- 

 times it is split behind into two tentacle-like projections. Morpho- 

 logically it is considered according to Cuvier, to be formed by the 

 united buccal tentacles and the upper tentacles or rhinophores. 

 The shell nearly always is present, but in some cases is rudimentary. 



(134) 



