MECUANISJI OF THE AJIPULLARIIDyE. 345 



Pachylahra, though an intUvidnal feature is seen in the develop- 

 ment of an upraised ridge on the right-liand edge. The orifice is 

 ahiiost as long as in Fachylabra, but in the two species examined 

 it does not extend so far backwards. 



It is impossible as yet to discuss tlie course of evolution in the 

 AmpullariidsB. It would seem, however, that the method of 

 respiration and the structure of the siphon and the respiratory 

 orifice exhibit a correlated a.nd progressive modification. At first 

 the siphon is short and almost functionless, the respiratory orifice 

 elongate. Then the siphon acquires aii intimate association with 

 the left-hand side of the pallial aperture which it assists to close 

 during siphonal respiration. It increases in length and can 

 become completely tubular. The i-espiratory orifice becomes 

 restricted in size and anteiior in position and possibly more 

 directly .approximated to the base of the siphon. 



D. The development of .the lung-cavity * from the branchial 

 chamber in the AmpullariidtB is comparable to the similar 

 phenomenon in Birgus latro and the terrestrial Brachyura and in 

 certain gobiid fishes, and the amphibious habits of these animals 

 are in general alike. Semper (8) and others have dealt with the 

 causes which may lead to the development of the amphibious 

 habit, so that there is no need to discuss this ma,tter in detail. 

 It might be pointed out that in the case of AmjmUaria the 

 development of a lung is apparently not necessarily connected 

 with the periodic drying-upof the sti-eams a.nd the sheets of Avater 

 in which the molluscs live. Amjndlaria, to judge from numerous 

 observations, is capable of spending a long period in a state of 

 reduced metabolism familiar to all students of molluscan aesti- 

 vation. This process is of course facilitated in operculate forms 

 by the retention of moisture in the tissues owing to the comjDlete 

 insulation of the animal by the operculum. The methods by 

 which the Ampullariidas are able to resist the annual periods of 

 desiccation are, therefore, probably the same as that of other tropical 

 molluscs. The significance of the development of the siphon is 

 quite a different matter. The development of the siphon is 

 probably coi-related with an increased tendency to live in streams 

 covered with a dense mat of surface vegetation in which the 

 short siphonal forms could not exist. In drawing attention to a 

 possible adaptationai value of the siphon, I do not Avish to assert 

 that it was necessarily adaptive in its origin, 



I am indebted to my friend Dr. Baini Prashad of the Zoological 

 Survey of India for permission to state that, so far as Pachylahra 

 is concerned, his views (arrived at independently) with regard to 

 the adaptationai value of the long siphon are the same as those 

 advanced above. 



* It is interesting to note tliat Brooks and McGlone (4), who have studied the 

 development of the North American A. depressa,^r\6L that the lung-cavity is de- 

 veloped as an infolding between the ridges from which the gill and osphradium 

 are developed, all three structures arising from practically the same area in the 

 embryonic mantle-cavity. 



