482 THE ANIMAL KINGDOM. 



water ; some of the last-named group are also found in brackish water, e.g. 

 Littorina, whilst others, such as Cyclostoma, are, like the stylommato- 

 phorous -Pulmonata, terrestrial. A few are parasitic : Entoconcha mirabilis 

 inhabits the perivisceral cavity of a Synapta (Holothurioided) ; in the genus 

 Eulima, which lives upon Holothurioidea> some species are ecto-, others 

 endo-parasitic. Stylifer is ecto- or pseudo-parasitically attached to Holo- 

 thurioidea, Echinoidea, and Asteroidea. Some are entirely pelagic, as the 

 Natantia and Phyllirhoe. The majority are reptant, but of these some few 

 can swim, as does, e.g. Oliva, by means of the epipodia. The majority are 

 vegetable feeders, the siphonostomatous Azygobranchia carnivorous. The 

 Anisopleura appear in the primary rocks, the existing genera Patella, Ttirbo, 

 Natica, and Pleurotomaria in the Lower Silurian. Though the number of 

 fossil forms is great, the extinct genera are not numerous. 



The Gastropoda Anisopleura are classified as follows (Ray Lankester) : 

 I. Streptoneura (= Prosobranchia minus Placophora). Visceral nerve-loop 

 twisted ; sexes separate. 



(1) Zygobranchia. Both ctenidia persistent or aborted. A left rudimentary 

 nephridium present in some instances. No accessory generative organs. The 

 visceral dome relatively small ; lying close upon the foot and co-extensive with its 

 prolongation in an aboral direction. Marine. 



Ctenidiobranchia, including Haliotidae and Fissurellidae ; Phyllidiobranchia, 

 including the Patellidae with secondarily developed gill-lamellae. 



(2) Azygobranchia. Left ctenidium, with corresponding osphradium and 

 right nephridium retained (see p. 479, and note). Osphradium often large and 

 lamellate = parabranchia. Hypobranchial and adrectal gland often present. 

 Shell usually large and spiral; operculum often present. Visceral dome large. 

 Anus on the right side. Radular teeth vary much in form. 



(a) Reptantia. Foot or mesopodium a creeping disc. Includes (i) Holo- 

 chlamyda ; with mantle edge entire ; with mouth, as a rule, at the end of a non- 

 introversible snout, i. e. rostriferous ; vegetarian ; marine, brackish, freshwater, or 

 terrestrial, e. g. Trochus, lanthina, Paludina, &c. : (2) Pneumonochlamyda, no cteni- 

 dium ; pallial chamber, a lung sac ; rostriferous ; terrestrial, e. g. Cyclostoma : (3) 

 Siphonochlamyda, a pallial siphon ; skull always spiral ; usually an operculum, horny, 

 and lamelliform ; rostriferous, or a proboscis ; mostly carnivorous ; marine, e. g. 

 Strombus, Cypraea, Conus, Murex, Bucdnum. 



(ft) Natantia (= Heteropodd). Pelagic; foot a swimming organ; carnivorous. 

 Atlantacea, with large visceral dome and shell : Carinariacea, visceral dome greatly 

 reduced; shell small, cap-like, hyaline; ctenidium projecting freely; mesopodium 

 with a sucker, fin-like ; metapodium pointed, forming posterior end of body : Ptero- 

 tracheacea, no shell; visceral dome, an oval sac imbedded in the posterior dorsal 

 region of body ; mesopodial sucker absent in female. 



II. Euthyneura. Visceral nerve loop not twisted, often very short; shell 

 generally light, little calcified ; often lost in adult ; operculum often present in 

 embryo but lost in adult ; radular teeth usually fine and similar : anus not so far 



