BOYCOTT: NOTE ON GENITALIA OF ACANTHINULA LAMEIXATA. 177 



were taken from among the largest of each batch, and the diameter of 

 the shells was between 2"o and 2-5 mm., mostly about 2 "2 mm. 

 These dimensions correspond with those given by Jeffreys and subse- 

 quent systematic writers^ and if the mature /^w^/Az/a is much larger it 

 seems to have escaped notice, (d) A seasonal fluctuation of the 

 required degree is unlikely and is fairly well excluded by the specimens 

 having been collected in February, June, July and October, {e) That 

 twenty abnormal specimens should have been taken from among 

 about 120 seems very improbable. 



The provisional conclusion, theretore, is that this apparatus is all 

 that lamellata usually has. Such an arrangement includes the really 

 essential organs and presents no particular functional difficulties 

 though it would require a good deal of ingenuity for two individuals 

 to effect simultaneous rather than successive fertilisation. The nor- 

 mal helicid penis is after all much the same thing structurally as the 

 vagina, and any protrusible tube would presumably do for an intro- 

 mittent organ. - Indeed, so far as I have been able to ascertain, no 

 one has seen lainellata in copula^ and it may be that it is self-fertilising. 

 None of the specimens showed eggs or spermatozoa nearer the 

 exterior than the hermaphrodite duct and they, therefore, throw no 

 light on this question. The organ called the spermatheca is so 

 identified on morphological grounds alone : I know nothing of its 

 function. 



The significance of genitalia so simple may be that the species 

 represents a primitive or decadent type, and its curious geographical 

 distribution in the north of England (with stragglers as far south as 

 Berkshire), Scotland and throughout Ireland may, perhaps, be taken 

 to favour such a view. An alternative, but not necessarily exclusive, 

 hypothesis suggests that a small snail may have simple organs because 

 there is not enough room for larger ones. On general grounds it 

 seems likely that such components as nervous ganglia and eggs would 

 be relatively larger in small than in large species ; if this proves to be 

 the case, it follows that some other parts must be reduced in size or 

 suppressed.^ There is too the highly significant fact that the smallest 

 complicated animals are relatively large while simpler organisms pass 

 by continuous gradations from submicroscopic to ultramicroscopic 

 dimensions and have an inferior limit in the neighbourhood of 

 molecular sizes. The smallest proctotrypid, trichopterygid or Vertigo 



1 Jeffreys (1862) o'og in. (=2"3 mm.) ; Williams (1888) 2i mm. ; Adams (1896) sj mm. 

 Swanton (1906) 2 to 2*3 mm. ; Geyer (1909) 2 to 25 mm. 



2 My attempts to keep lainellata alive in captivity have miserably failed. 



3 cf. the suggestion of H. WatscMi {Anvah of Natal Museum, vol. iii. (1915), p. 229) that 

 the absence of large accessory organs on the genital ducts of Tcstacella is due to the great 

 development 01 the buccal mass combined with the necessity of a slender habit of body. 



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