MOLLUSCA. 161 



Order I. SCAPHOPODA. 



SOLENOCONCILE. PROSOPOCEPIIALA. ClRROBRANCHIA. 



Head rudimentary ; a pair of horny jaws ; mouth surrounded 

 by many filiform tentacles. No eyes. Dioecious. 



There is no heart nor branchiae. The foot is tbree-lobed. The 

 shell is slender, conical, curved, and perforate at the apex, and 

 the aperture is round. The animal is attached to the shell at the 

 hinder part. The young are free-swimming, propelled by vi bra- 

 tile cilia. 



There is only one family, containing, according to Schmarda, 

 about 50 living and 125 fossil species ; they mostly occur in the 

 Devonian formation. 



Dentaliida (Tooth-shells). 

 Dentalium. 

 Entalis = Antalis. 

 Gadila. 



Order II. OPISTHOBRANCHIATA. 



Eespiration aquatic, effected by arborescent or fasciculate 

 branchiae, more or less exposed, rarely absent ; and placed pos- 

 teriorly. With or without a shell in the adult. Hermaphrodite. 

 Larva with a velum and shell. 



The shell is rudimentary or absent ; but in many cases it is well- 

 formed, and is always enclosed in the mantle. Tentacles are 

 generally present, and are mostly non-retractile. The branchias 

 are either covered or uncovered ; in the latter case they are occa- 

 sionally capable of being withdrawn into one or more branchial 

 cavities. The nervous system is well developed ; aud they have, 

 with few exceptions, two eyes, which are placed behind the ten- 

 tacles. 



Most of the species are littoral, swimming or creeping about 

 amongst sea-weed, but a few are pelagic. They are carnivorous. 

 One of the commonest species on our shores is known as the sea- 

 hare (Aplysia depilans} ; it is said to emit a violet-coloiyed fluid 

 when molested, due to the presence of iodine. r 'Sea-slugs seemSto 

 be a general term applied to several species or even genera. The 

 nearest external approach to slugs is perhaps best shown in JEolis 

 and the Abranchia. 



Certain worm-like marine animals forming the genus Neomenia 

 ( = Solenopus} compose the i( order" Telobranchiata of Koren and 

 Danielssen. 



