182 



THE GASTROPODA 



often reduced ; the latter may be partially covered over, or interral, 

 or even absent. There is never an operculum in the adult, except 

 in Amphibola, and an operculum is only found during development 

 in the Auriculidae, Siphonariidae, and Oncidiidae, all of which are 

 marine forms. In the pallial cavity the interior wall of the mantle 

 is traversed by vascular arborisations (Fig. 86, X), and thus con- 

 stitutes a pulmonary organ adapted for breathing air. In the 

 Janellidae the pulmonary cavity is prolonged into tine respiratory 

 canaliculi (Fig. 90, tr), and thus becomes a tracheal lung. It is 

 much reduced in the Oncidiidae, and in Ancylus and the Vaginulidae 

 it disappears as a consequence of the complete abortion of the pallial 

 cavity^-Jln some rare cases the pulmonary cavity may be filled 

 with water, and then its wall may give rise to a secondary branchia 

 which is not the equivalent of a ctenidium (Siphonaria, Fig. 174, 

 III). In other cases the inferior pallial lobe, situated beneath the 

 pulmonary orifice of the Basommatophora, may be transformed 

 into a branchia (Planorbidae, Figs. 89, <-/, and 1 75, br). \ The auricle 

 of the heart is usually anterior (Fig. 86), as is the ckte in the most 



archaic Opisthobranchs, and it is only 

 in the excessively detorted forms such 

 as Testacella and the Oncidiidae that 

 the ventricle lies in front of the 

 auricle. The kidney usually has a 

 more or less elongated duct or 

 " ureter " (Stylommatophora, Fig. 

 86, V). In the nervous system, as a 

 rule, all the ganglia are concentrated 

 round the oesophagus and are closely 

 apposed to one another (Fig. 146), but 

 this is not the case in some archaic 

 Basommatophora such as Ckilina, 

 Auricula, Latia (Fig. 96). In the 

 Auriculid Pythui, the spermiduct re- 

 tains the character of an open ciliated 

 groove leading from the hermaphro- 

 dite aperture to the penial orifice 

 (Fig. 171, ci) : in other Auriculidae 



Reproductive apparatus of Pythia. this grOOVC is Simply closed to form 



a canal extending from the herma- 



mnc, mucous gland ; pe, penis ; ret, re- other PulmonatCS there IS 110 longer 

 tractor muscle of penis ; r.s, recepta- ., , / i .1 



culm., aemiuia; *;>; spermiduct ; MM, a common genital orifice, but the 

 (TrtTpiaS ; ve ' 3 ' 8eminal vesicle ' hermaphrodite duct bifurcates to 



form a distinct oviduct of greater 



or less length, and the primitive hermaphrodite aperture becomes 

 the female orifice. As a result of secondary changes, the orifices 



