8oo 



CHAPTER XXXIX. 



DIVISIONS OF GASTROPODA— continued. 



BRANCHIOGASTROPODA-^/»^. 



Order II. Opisthobranchiata. 



The Opisthobranchiate Gastropods are distinguished by the fact 

 that the branchiae are situated behind the heart, and the auricle is 

 behind the ventricle of the heart. The gills are not usually con- 

 tained in a special branchial chamber, and are commonly more or 

 less exposed to view. The shell is wanting or present, being in the 

 latter case often rudimentary. The sexes are united in the same 

 individual. 



The Opisthobranchiata, or " Sea-slugs," may be divided into two 

 sections, the Tectibranchiata and Nudibranchiata, according as the 

 branchiae are protected or are uncovered. In the Nudibranchiate 

 forms, the branchiae, when differentiated, are placed externally on 

 the back or sides of the body, and the animal, in the adult condi- 

 tion, is destitute of a shell. Owing to their want of hard structures, 

 no traces of the Nudibranchs have ever been detected in the fossil 

 condition. On the other hand, in the Tectibranchiate forms, the 

 mantle more or less extensively covers the gills, and very generally 

 secretes a shell, which may be so greatly developed as to entirely 

 enclose the animal when withdrawn within it. Owing to their pos- 

 session of a shell, the majority of the families of the Tectibranchs 

 are known to occur in the fossil state, the earliest forms appearing 

 in the Carboniferous rocks. The number of fossil Tectibranchiates 

 is, however, very limited as compared with that of the Prosobranchi- 

 ate Gastropods, the following four families being those w T hich have 

 yielded fossil representatives : — 



Family i. Act^onid^: (Tornatellilve). — In this family the 

 shell is spiral or convoluted ; the aperture is long and narrow, 

 rounded or sometimes channelled in front ; and the columella is 



