Fig. 266.—Neomenia carinata Tullb.: a, anus; gr, ventral groove; m, mouth.
Fig. 267.—Chaetoderma nitidulum Lov.: a, anus; m, mouth. × 3.
Fam. 2. Chaetodermatidae.—Body cylindrical, no ventral groove, liver a single sac, kidneys with separate orifices into the branchial cloaca, two bipectinate ctenidia. Single genus, Chaetoderma (Fig. [267]).
Order II. Prosobranchiata
Visceral loop twisted into a figure of 8 (streptoneurous), right half supra-intestinal, left half infra-intestinal; heart usually in front of the branchia (ctenidium), which is generally single; head with a single pair of tentacles; animal dioecious, usually marine, more or less contained within a shell, operculum generally present. Cambrian to present time.
Sub-order 1. Diotocardia.—Heart with two auricles (except in the Docoglossa and Helicinidae), branchiae bipectinate, front end free; two kidneys, the genital gland opening into the right (except in Neritidae); nervous system not concentrated; no proboscis or siphon, penis usually absent.
(a) Docoglossa (p.[ 227]).—Heart with a single auricle, ventricle not traversed by the rectum, visceral sac not spiral, shell widely conical, non-spiral, no operculum; radula very long, with few hooked teeth in each row.
Fam. 1. Acmaeidae.—Left ctenidium alone occurring, free on a long stalk. Cretaceous——. Principal genera: Pectinodonta, front part of head much produced, radula 0 (1. 0. 1.) 0; Acmaea (= Tectura), with sub-genera Collisella and Collisellina, no accessory branchial ring, shell closely resembling that of Patella, but generally with a distinct internal border; Scurria, accessory branchial ring on the mantle.