934 MOLLUSCA AND BKACHIOPODA 



branchi.e and of oral tentacles ; but their nature can only be cer- 

 tainly determined by further observation, which is rendered difficult 

 by the opacity of the valves. By keeping a supply of fish, however, 

 with these embryos attached, the entire history of the development 

 of the fresh- water mussel may be worked out. 1 



In certain members of the class Gastropoda the history of em- 

 bryonic development presents numerous phenomena of great interest. 

 The eggs (save among the terrestrial species) are usually deposited in 

 aggregate masses, each inclosed in a common protective envelope or 

 nidamentum. The nature of this envelope, however, varies greatly ; 

 thus, in the common Limnceus stagnalis, or ' water-snail,' of our ponds 

 and ditches it is nothing else than a mass of soft jelly, about the size 

 of a sixpence, in which from fifty to sixty eggs are imbedded, and 

 which is attached to the leaves or stems of aquatic plants ; in the 

 Buccinum undatum, or common whelk, it is a membranous case, 



p. ad 



FIG. 710. A, Glochidium immediately after it is hatched : ad, ad- 

 ductor ; sh, shell ; by, byssus-cord ; s, sense-organs. B, the same 

 after it has been on the fish for some weeks : br, branchiae ; auv, 

 auditory sac; /, food; a.ad and p. ad, anterior and posterior 

 adductors ; al, mesenteron ; mt, mantle. 



connected with a considerable number of similar cases by short stalks, 

 so as to form large globular masses which may often be picked up on 

 our shores, especially between April and June ; in the Pwrpura 

 lapillus, or ' rock-whelk,' it is a little flask-shaped capsule, having 

 a firm horny wall, which is attached by a short stem to the surface 

 of rocks between tide marks, great numbers being often found 

 standing erect side by side ; whilst in the JSTudibranchiate order 

 generally (consisting of the Doris, JSolis, and other ' sea-slugs ') it 

 forms a long tube with a membranous wall, in which immense 

 numbers of eggs (even half a million or more) are packed closely 

 together in the midst of a jelly-like substance, this tube being disposed 

 in coils of various forms, which are usually attached to seaweeds or 

 zoophytes. The course of development, in the first and last of these 

 instances, may be readily observed from the very earliest period down 



1 See the Kev. W. Houghton, ' On the Parasitic Nature of the Fry of the Ano- 

 donta cygnea,' in Quart. Journ. Microsc. Sci. n.s. vol. ii. 1861, p. 162, and especially 

 Balfour, op. cit. pp. 220-223. On the embryonal byssus-gland of Anodonta, see 

 J. Carriere, Zoolog. Anzeig. vii. p. 41. 



