GASTROPODA 495 



Key to the orders of Gastropoda: 



Oi Laud aud fresh-water snails ; lungs, and not gills, present ; no oper- 

 culum 2. PULMONATA 



©a Mostly aquatic snails with either ctenidia or adaptive gills. 



6i Cteuidia posterior in position or adaptive gills present ; shells small 



or wanting ; all marine 1. Opistiiobranchiata 



6a Ctenidia at forward end of bodj' ; shell well developed ; operculum usu- 

 ally present ; mostly marine 3. Prosobranchiata 



Order 1. OPISTHOBRANCHIATA.* 



Sea slugs. Marine snails, in which the mantle and shell are either 

 entirely wanting, or when present are usually small. The body is asym- 

 metrical, the kidney and genital pores and usually the anus being on the 

 right side; in certain nudibranchs the anus is (secondarily) median and 

 posterior in position. The shell, when present, is usually a spiral. In the 

 nudibranchs the ctenidia and osphradia are absent, respiration being 

 effected either by the general surface of the body or by so-called adapt- 

 ive gills, which are projections of the dorsal body wall (Fig. 765). The 

 foot has usually a broad sole, but in the pteropods is highly modified, the 

 epipodia alone being well developed, forming a pair of fins, by means of 

 which the animal swims (Fig. 759). In many others also the epipodia 

 form extensive folds which cover the back or are used in many cases in 

 swimming (Fig. 769). 



The nervous system is compact, the ganglia being usually near 

 together just behind the pharynx; the pleurovisceral connectives are not 

 crossed except in Acteon. Two pairs of tentacles are present, the anterior 

 pair being sometimes absent, the posterior pair (rhinophores) having 

 often a lamellar stmcture. The eyes are at the base of these posterior 

 tentacles. The radula contains numerous teeth, and the jaw is composed 

 usually of two pieces. The animals are hennaphroditic, the common 

 gonad being protandric ; in the lower opisthobranchs it is joined with the 

 genital pore by a single duct, but in the higher ones a distinct oviduct 

 and vas deferens are present. The auricle lies back of the ventricle and 

 receives blood from the gills, which are also mostly posterior in position. 



The opisthobranchs are mostly littoral animals living under stones 

 or among seaw^eed, and feeding principally on animal food, and many are 

 brightly colored. The pteropods as well as representatives of several 

 other groups are pelagic. The order contains over 2,300 species, grouped 

 in 2 suborders. 



Key to the suborders of OpisthohrancJiiata : 



tti Shell usually present ; gill in the mantle cavity 1. Tectibranchiata 



Oa Shell and mantle absent; adaptive gills on the back 2. Nudibranchiata 



* See "Manua: of Conchology," Vol. 15 and 16, by H. A. Pilsbry, 1893-1896. 



