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 NudibrancJnata 

 and Pulmonatu 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 modification*. Tin- rroup is primarily divided into three 

 sections, whose characteristic features are well expressed in their 

 names: 



I. ( 'i IMI \i.\-rn>i \ : II. ad with a fleshy disc or shield. (Bulla, 

 etc.). 



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



III. NCI A- i-ii.i \ : no head disc; back protected by a large 

 shield or n.taiiin, and by a true mantle- and shell. (Umbrella, etc.) 

 iir>t of these sections will now be considered. 



I. Ti m;i: \\rmATA < 'i:rn AI.ASPIDEA. 



"All of then- animal:* are characteri/ed by the presence of ahead- 

 disc, di-tinct Jr.. m th. Lack. Lcarin;: t lie sessile eyes and i he tentacles 

 : i present. Tin- .1 is to be a tactile organ. It varies 



in many \\a\>. fu mi -hi ng good characters for classification. Some- 

 it behind into two tentacle-like projections. Morpho- 

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

 united bu-c:il tentacles and the upper tentacles or rhinophores. 

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



(134) 



