THE GASTROPODA 



173 



FAMILY 2. PNEUMONODERMATIDAE, Gray. Shell and mantle absent ; foot 

 shorter than the visceral mass, and with a much-reduped ventral surface ; 

 parapodia highly developed and fin- shaped ; pharynx evaginable, with 

 suckers. Genera Dexiobranchaea, Boas ; suckers independent ; no terminal 

 posterior branchia. Pmumonoderma, Cuvier ; suckers united on two lobes ; 

 a quadriradiate terminal branchia. Sponyiobranchaea, d'Orbigny ; a 

 simple annuliform terminal branchia. Schizobracliium, Meisenheimer ; 

 acetabuliferous appendages ramified. FAMILY 3. CLIONOPSIDAE, Costa. 

 No buccal appendages or suckers ; a very long evaginable proboscis ; a 

 quadriradiate terminal branchia. Genus Clionopn^ Troschel. FAMILY 4. 

 NOTOBRANCHAEIDAE, Pelseneer. Posterior branchia triradiate. Genus 

 Notobranckaea, Pelseneer. FAMILY 5. THLIPTODONTIDAE, Kwietniewski. 

 Head very large, not marked off from the body ; neither branchia nor 

 suckers ; fins situated near the middle of the body. Genus Thliptodon, 

 Boas. FAMILY 6. CLIONIDAE, Gray. No branchia of any kind ; a 

 short evaginable pharynx, bearing paired conical buccal appendages or 

 " cephalocones." Genera Clione, Pallas. Paraclione, Tesch. FowUrina, 

 Pelseneer. FAMILY 7. HAI.OPSYCHIDAE, Pelseneer. No branchia ; two long 

 and branched buccal appendages. Genus Halo-psyche, Boas (Fig. 156). 



The last six families form the group formerly known as the " Gymno- 

 somatous Pteropoda," characterised by the absence of the mantle and shell, 

 the reduction of the ventral surface of 1X 



the foot, and the fin-shaped parapodia 

 placed at the anterior end of the body. 

 They are all pelagic. 



TRIBE 3. PLEUROBRANCHOMORPHA. 



In these Tectibranchs there are 

 two pairs of tentacles. The foot is 

 devoid of parapodia. There is no 

 pallial cavity, but there is always a 

 single ctenidium situated on the right 

 side and occupying the space between 

 the mantle and the foot. The genital 

 duct is diaulic, without an open 

 seminal groove ; the male and female 

 apertures are contiguous. The vis- 

 ceral commissure is short, and re- 

 sembles that of the Nudibranchs in 

 showing a tendency to the fusion of 

 the supra-intestinal and sub-intestinal 

 with the pleural ganglia, and the 

 concentration of all the ganglia on 

 the dorsal side of the oesophagus. 



FAMILY 1. TYLODINIDAE, Mazza- 

 relli. Shell external and conical ; the 

 anterior tentacles form a frontal veil ; the ctenidium extending only over 

 the right side ; a distinct osphradium. Genus Tylodina, Kafinesque ; 



Fio. 157. 



Pleuroltranckaea meckeli, dorsal aspect. I, 

 posterior tentacle or rhinophore ; 1 1, mantle ; 



prebranchial gland ; VII, genital (herma- 

 phrodite) orifice ; VIII, the fused anterior 

 tentacles ; IX, expanded proboscis. 



