THE GASTROPODA 151 



apophysis. Genus Hydrocena, Parreys ; from Dalmatia. FAMILY 23. 

 PROSERPINIDAE, Fischer. Differs from the two last families in not 

 having an operculum. Genus Proserpina, Gray ; from Central America. 



ORDER 2. Pectinibranchia. 



These are Streptoneura with a somewhat concentrated nervous 

 system ; without a labial commissure, except in Paludina and Ampul- 

 lina. The nerve-collar is situated behind the buccal bulb, except in 

 Ampullaria. There is a single well- differentiated, independent, and 

 often pectinated osphradium. The eye is always closed, and the 

 internal cornea (pellucida) is extensive. Each otocyst contains a 

 single otolith, except in some forms of Taenioglossa devoid of a pro- 

 boscis, e.g. Paludina, Valvata, Ampullaria, Cydophorus, Typhobia, 

 Bt/thoceras, Nassopsis, certain Cerithiidae, etc. The central tooth of 

 the radula is single or absent. There is no longer any trace of 

 bilateral symmetry in the circulatory, respiratory, and excretory 

 organs, the topographically right half of the pallial complex having 

 completely disappeared. The heart has only a single auricle that 

 of the morphologically right side and is not traversed by the rec- 

 tum. The ctenidium is monopectinate and attached to the mantle 

 throughout its length, except in Adeorbis and Valvata, the latter 

 genus being the only Pectinibranch with a bipectinate ctenidium. 

 The single kidney usually opens directly by a slit-shaped aperture 

 (but exceptionally by a ureter in Paludina, Ci/clophorus, and Valmta), 

 and never serves for the passage of the sexual products. The 

 gonad always has a separate orifice of its own. The male generally 

 has a penis (Fig. 44, A, h). 



The Pectinibranchia are divided into two sub-orders Taenio- 

 glossa and Stenoglossa. 



SUB-ORDER 1. TAENIOGLOSSA. 



In these Pectinibranchs the radula bas normally three teeth on each 

 side of the median tooth, viz. one lateral and two marginals (Fig. 2, B ; 

 74, B). The stomatogastric ganglia are situated behind the buccal mass, 

 and are united to the cerebral centres by long connectives which are in 

 part recurrent and deeply situated. The salivary ducts, when sufficiently 

 long, traverse the nerve-collar. The oesophagus is nearly always devoid 

 of an unpaired gland. Usually there is neither a proboscis nor a siphon. 

 The sub-order includes two distinct groups or tribes, which are 

 respectively creeping and swimming forms, namely, the Platypoda and 

 Heteropoda. 



TRIBE 1. PLATYPODA. 



Normal Taenioglossa, but slightly modified, and of creeping habit. 

 The foot is flattened ventrally, at all events in its anterior part (Strom- 

 bidae). The otocysts are situated close to the pedal nerve-centres. Acces- 



