20 ORIGIN OF LAND OPERCULATES chap. 



Pulmonata^ as a whole, as derived from the Tectibranchiate 

 Opisthobranchs, while others^ go further and regard the Stylom- 

 matophora as derived directly from the Basommatophora. 



Origin of the Land Fauna 



Gasteropoda. — (1) Operculate. On a priori grounds, one 

 might predict a double origin for land operculates. Marine 

 species might be imagined to accustom themselves to a terres- 

 trial existence, after a period, more or less prolonged, of littoral 

 probation. Or again, fresh-water species, themselves ultimately 

 derived from the sea, might submit to a similar transformation, 

 after a preliminary or intermediate stage of life on mudbanks, 

 wet swamps, branches overhanging the water, etc. Two great 

 families in this group, and two only, seem to have undergone 

 these transformations, the Littorinidae and the Neritidae. The 

 derivation of almost all existing land operculates may be referred 

 to one or other of these groups. 



Fig. 16. — Two rows of the radula of Littorina littorea L., x 72. 



The power of the Littorinidae to live for days or even weeks 

 without being moistened by the sea may be verified by the most 

 casual observer. In the tropics this power seems even greater 

 than on our own shores. I have seen, in various parts of 

 Jamaica, Littorina muricata living at the top of low cliffs among 

 grass and herbage. At Panama I have taken three large species 

 of Littorina (yaria^ fasciata, pulchra^, on trees at and above 

 high-water mark. Cases have been recorded in which a number 

 of L. mu7ncata, collected and put aside, have lived for three 

 months, and L. irrorata for four months.^ These facts are 

 significant, when we know that the land operculates almost 

 certainly originated in a tropical climate. 



1 E.g. Bergh, Zool. Jahrb. v. p. 1 f. 



2 Calkins, Ajner. Nat. xi. p. 687. 



