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 Harpidce among the Rhacliiglossa, 

 the mantle bends back over the columellar 

 lip of the shell. In the Margimllidm, it 

 covers a large part of the outer surface, and 

 the same is the case in Pynda among the 

 Taenioglossa, in most Gyprceidcc and in the 

 Lamellaridcc. In Lamellaria, the shell is 

 cqmpletely grown over by the mantle. In 

 Stilifer among the Eulimidce also, the shell 

 is more or less covered by the mantle. 



The edge of the mantle may be fringed 

 or notched, or (Cypraeidce) provided with 

 wart -like, tentacular, or branched ap- 

 pendages. 



e.. 



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 

 FIG. 54. Testaceiia haiiotidea (after protruding spi rally -coiled visceral 



Lacaze-Duthiers). A, right view ; b, enormous dome and } mant l e f ^ enclosing 



pharynx evaginated through the buccal cavity, __;*._ ^ ^ 



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 : a, b, the two pairs of tentacles ; 



c, the latero-ventral groove ; <?, the latero- 

 dorsal groove ; e, shell. 



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 



