THE GASTROPODA 163 



characteristically prosobranchiate. In Cannaria the shell is uncoiled 

 and rudimentary, and there is no operculum. Pterotrachea has neither 

 shell nor tentacles and is opisthobranchiate. Finally, Firoloida has lost 

 the ctenidium. 



FAMILY 1. ATLANTIDAE, Rang. Visceral sac and shell spirally coiled 

 in one plane ; foot divided transversely into two parts, the posterior part 

 bearing an operculum with a sinistral coil (Fig. 48), while the anterior 

 part forms a fin provided with a sucker. Genera Oxygyrus, Benson ; 

 shell capable of containing the entire animal, carinated only on the last 

 whorl and near the aperture. Atlanta, Lesueur; shell capable of con- 

 taining the whole animal, carinated throughout ; aperture with fissures 

 (Fig. 141). FAMILY 2. CARINARIIDAE, Grasset. Visceral sac and shell 

 conical and small in proportion to the rest of the body, which cannot be 

 withdrawn into the shell ; foot elongated, fin-shaped, with a sucker but 

 without an operculum. Genera Carinaria, Lamarck (Fig. 142). Cardio- 



FIG. 143. 



Pterotrachea mutica, seen from the right side, a, pouch for the reception of the snout when 

 retracted ; br, ctenidium ; c, pericardium ; g, cerebral ganglion ; g', pedal ganglion ; i, intestine ; 

 mt, posterior part of the foot ; n, so-called visceral nucleus ; oc, cephalic eye ; ph, pharynx ; pr, 

 fin-like anterior part of the foot ; v, oesophagus ; vi, osphradiuin ; z, caudal appendage. (From 

 Lankester, after Keferstein.) 



poda, d'Orbigny (Fig. 142, C, D). FAMILY 3. PTEROTRACHEIDAE, Gray. 

 Visceral sac very much reduced, without shell and mantle ; anus on the 

 posterior part of the body ; foot provided with a sucker in the male 

 only. Genera Pterotrachea, Forskal ; no tentacles ; a ctenidium present ; 

 a filiform appendage at the posterior extremity of the foot (Fig. 143). 

 Firoloida, Lesueur ; tentacles present, but no ctenidium and no posterior 

 appendage to the foot. Pterosoma, Lesson. 



SUB-ORDER 2. STENOGLOSSA. 



Pectinibranchs in which the nervous system is much concentrated 

 and always zygoneurous. The perioesophageal nerve -collar is always 

 posterior to and is not traversed by the salivary glands. The stomato- 

 gastric ganglia are situated close to the cerebral nerve-centres and far 

 behind the buccal mass, the last-named organ being greatly reduced. A 

 well-developed proboscis, an unpaired oesophageal gland (the gland of 

 Leiblein or poison-gland), a pallia) siphon, and a penis are always present. 

 The osphradiuin is bipectinate. The radula is narrow, and in the 

 majority of genera (Rachiglossa) has a single lateral on each side of the 

 median or rachidian tooth : in the remainder of the group (Toxiglossa) 

 there is no median tooth, and the radular formula is therefore 1.0.1. 

 The sub-order is accordingly divided into two tribes. 



