44 



COMPARATIVE ANATOMY 



CHAP. 



The Monotocardia have even been grouped, according to the presence or absence 

 of a siphon, into the Siphoniata or Siphonostomata, and the Asiphoniata or Holo- 



stomata ; but this classification is artificial, 

 since siphons are sometimes present and 

 sometimes absent in forms which are un- 

 doubtedly nearly related. 



In most Monotocardia, the shell is not 

 outwardly covered by the mantle, but in 

 some groups, the edges of the mantle bend 

 back over the shell, and finally grow over 

 it to 'such an extent as to unite above it. 

 The external shell in such cases becomes 

 an internal shell. 



In the Jlnr/iiif'f among the llhuchiyJo^i, 

 the mantle bends back over the columellar 

 lip of the shell. In the Miinjin^Iliilii'. it 

 rovers a large part of the outer surface, and 

 the same is the case in Pyrula among the 

 T<i<'iiin<jli,s*i, in most Cyprceidce and in the 

 L'i,,u/liiridce. In Lamcllaria, the shell is 

 completely grown over by the mantle. In 

 Stilih r among the En.tiinidcfi also, the shell 

 is more or less covered by the mantle. 



The edge of the mantle may be fringed 

 or notched, or (Cyprneidce) provided with 

 \vart-like, tentacular, or branched ap- 

 pendages. 



2. Pulmonata. 



In the Pulmonata, the arrange- 

 ments of the mantle fold and visceral 

 dome and of the shell, which is in- 

 timately connected with them, are of 

 great interest. We have, on the one 

 hand, forms such as Helix, with large 



Testaceiia haiiotidea (after protruding spi rally -coiled visceral 

 Lacaze-Duthiers). A, right view ; b, enormous dome and j mantle fold enclosing 



pharynx evagmated through the buccal cavity, . . , , 



a spacious cavity ; on the other, 

 forms such as Oncidium, without 

 distinct visceral dome or mantle fold 

 and without shell. Between these 

 two extremes there are numerous 

 transition forms ; indeed, complete 

 series of such forms may be found 



even within some of the natural divisions of the Pulmonata. The 



following are a few characteristic types. 



Helix (Figs. 12 A, p. 9 ; 72, p. 75). The visceral dome is large and spirally 

 coiled, and is covered by a spiral shell sufficiently large to shelter with ease the whole 

 body. The mantle fold covers a cavity lying anteriorly to the visceral dome (pul- 

 monary cavity). Its free thickened glandular edge unites with the nuchal integument 



FIG. 54. 



carrying on its surface the radula (a) ; c, open- 

 ing of the pharynx into the oesophagus ; d, 

 position of the genital aperture ; e, latero-dorsal 

 groove along the body ; /, latero-ventral groove ; 

 g, mantle, rudiment of the visceral dome. B, 

 dorsal view : o, b, the two pairs of tentacles ; 

 c, the latero-ventral groove ; d, the latero- 

 dorsal groove ; e, shell. 



