VII 



MOLLUSCA SYSTEMATIC REVIEW 



11 



a. Pteropoda thecosomata. These are nearly related to the Cephalaspidea, 

 and possess a mantle, mantle cavity, and shell. The head is not distinct, and has 

 only one pair of tentacles. The fins, at their anterior edges, are fused over the mouth; 

 the anus lies to the left. Fam. Limacinidse. An external calcareous shell, with 

 left-handed or sinistral twist, and a spiral operculum. Anus to the right (Linia- 

 [Fig. 16], Peraclis). Fam. Cavoliniidse. External symmetrical shell (Clio, 

 Cavolinia). Fam. Cymbuliidse. Internal cartilaginous shell (Cymbulia, Cymbuli- 

 opsis, Gleba). The Thecosomata feed chiefly on small Protozoa and Algae. 



CL 



1 



FIG. Iti. Limacina Lesueuri (dorsal aspect, 

 after Pelseneer). 1, Penis ; 2, fin (parapo- 

 dium) ; 3, seminal furrow ; 4, mantle process 

 ("balancer"); 5, visceral dome; 6, head with 

 two tentacles and the seminal furrow 3. 



FIG. 17. -Pneumoderma (diagram from the 

 right, after Pelseneer). 1, right evaginated 

 process bearing hooks (hook sac) ; 2, proboscis ; 

 3, right buccal tentacle ; 4, position of the right 

 nuchal tentacle ; 5. right tin (parapodium) ; 6, 

 seminal furrow ; 7, genital aperture ; 8, position 

 of the jaw ; 9, ventral proboscidal papilla ; 10, 

 right buccal appendage provided with suckers ; 

 11. head ; 12, aperture for penis ; 13, right an- 

 terior pedal lobe ; 14, anus ; 15, posterior pedal 

 lobe ; 16, ctenidium ; 17, posterior adaptive 

 gill ; d, v, o, p denote dorsal, ventral, anterior, 

 and posterior. 



17 



b. Pteropoda gymnosomata. These are nearly related to the Anaspidae. 

 They have no mantle, mantle cavity, nor shell. The head is distinct, and carries 

 two pairs of tentacles. The fins are separate ; the anus lies to the right. Fam. 

 Pneumodermatidse. One ctenidium to the right (Dcxiobranch<ea, Spongiobranchcea, 

 Pif nrnod' rma [Fig. 17]). In the last two genera there is an adaptive posterior gill 

 as well. Fam. Clionopsidae and Notobranchseidse. Xo ctenidium, but a posterior 

 adaptive gill. Fam. Clionidse. Xeither ctenidium nor adaptive gill. All Gymnoso- 

 mata are carnivorous, feeding principally on Thecosomata. 



Sub-Order 2. Ascoglossa. 



This sub-order is characterised by the fact that the worn-out teeth of the long 

 narrow radula, which consists of a single row of dental plates, are preserved in a sac 



