NAKED-GILLED SUBGROUP 3323 



can be filled or emptied at the will of the animal. Although this, in the main, is 

 the principle of the circulation in most Opisthobranchs, one branch of the order 

 possesses no special breathing organ, respiration being effected through the naked 

 skin of the body. The Opisthobranchia may be divided into three principal subor- 

 ders, namely, Nudibranchiata, Tectibranchiata, and Pteropoda. 



SUBGROUP SUBORDER Nudibranchiata 



The naked-gilled Gastropods constitute a large assemblage of extremely beauti- 

 ful mollusks, of remarkable shape and often brilliant coloration. The distinguish- 

 ing characteristic of the typical forms consists in the breathing organs being exposed 

 on the back of the animal, and not protected by the mantle. Other groups, how- 

 ever, are classed within this suborder, in which either the position or character 

 of the respiratory organs is different. The gills may be situated on each side of 

 the body between the back and the foot, or respiration be effected by the ciliated 

 surface of the body. For these and other reasons the Nudibranchs have been ar- 

 ranged in four groups, Anthobranchiata, Inferobranchiata, Polybranchiata, and 

 Pellibranchiata. A fifth group, Parasita, has also been proposed for a very curious 

 mollusk (Entoconcha) , which lives parasitic within an echinoderm {Synapta). Nudi- 



Acanthodoris pilosa (magnified). 



branchs are found in all parts of the world, and are most abundant in depths where 

 seaweeds and corallines flourish, although they are also found at low water or even 

 between tide marks. A few species have been dredged at great depths. Some even 

 pass their life in the open sea, attached to floating seaweeds, or swim about freely 

 like Pteropods or Heteropods. 



The animals belonging to this group are characterized by their sym- 



bran h' t raetr i ca ^ form, and the position of the vent. This is placed upon the 



posterior or central part of the back and surrounded by the branching 



gills, which are more or less contractile. The body is protected above by a large 



dorsal shield of a spongy texture, and made more or less rigid by calcareous spicules 



which are embedded in it. The tentacles are dorsal, and retractile through holes 



