MOLLUSCS. 



295 



aquatic and some of the terrestrial species, while the latter contains only the Opistho- 

 branchs and Pulmonifera. 



Order I. — OPISTHOBRANCHIATA. 



This group is exclusi-\-ely marine, and is composed of forms with a large foot, while 

 the visceral hump, so characteristic of most gasteropods, is very small, or wanting. 

 The name has reference to the fact that the gills are placed behind the heart, winch is 

 but another statement of the fact that the torsion of the body has not been carried to 

 its full extent. In the adult stage some are provided with a shell, while in others this 

 is lacking; Imt all, without exception, have a shell in their earlier stages. 



Fig. 332. — Circulation in Pieurobranchus aurlantiacus, showing posterior position of tlie gills; a, mouth; 

 b, gill; g, renal opening; h, heart; v, veins. 



In some, gills are present, and may either project freely into the water or be con- 

 cealed in the mantle cavity, wliile others liave no S]iecialized respiratory organs. The 

 position of the gilLs, mentioned above, has also its effect upon the circulatory organs, 

 and hence the auricle is here behind the ventricle of the heart. The vent is upon the 

 side of the body, and the sexes ai-e united in the same individual. Two well-marked 

 sub-orders may be distinguished. 



Sitb-OrDER I. — NUDIBRANCHIATA. 



Possilily no grouj) of molluscs possesses more beautiful fonns, or affords more in- 

 stances of protective reseml)lances, than does that which has received tlie name Nudi- 

 branchiata. This term, which means naked 

 gills, is very appropriate, for these organs, 

 when present, are not enclosed in a special 

 respiratory cavity, but project freely into the 

 surrounding medium, and are borne either on 

 the back or on the sides of the animal. From 

 the fact that in the adult stage no shell is 

 jn-esent, these forms are frequently termed, 

 in more common paiiance, naked molluscs. 

 In the young stage a sliell is jiresent. This embryonic shell, which is formed and 

 acquires a spiral or nautiloidal form before the young leaves the egg, disappears with 

 growth. It is transparent, and the young animal can close the aperture with an oper- 

 culum, wliile at other times it jirojects from the ojiening a ciliated velum, with which 



Fic. ;W3. — Larva of Kntoconcka 



