934 



MOLLUSCA AND BRACHIOPODA 



branchia? 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 

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

 thus, in the common Limncnus staynalis, 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, ;md 

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

 Sucdnum w/ndatum, or common whelk, it is a membranous case, 





at/ 



F!(T. 710. A, Glochidium immediately after it is hatched : ail, ad- 

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

 after it has been on the fish for some weeks : b>\ branchiae ; am , 

 auditory sac; f, food; ft. ad and p. ad, anterior and posterior 

 adductors; al, inesenteroii ; tut, mantle. 



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

 so as to form large globular masses which may often lie 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 Nudibranchiate order 

 gem-rally (consisting of the Doris, Eolis, 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) arc packed closely 

 together in the midst of a jelly-like substance, tin's t ube bring disposed 

 in coils of various forms, which are usually attached 10 sea \\crds or 

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

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



- I. In- I!i-v. \V. Houghton, ' On the Parasitic Nature of the Fry of the A)u>- 

 il,inln cygnea,' in (Jimrl. Journ. Microsc. Sci. n.s. vol. ii. 1801, p. 162, and especially 

 Balt'our, <>i>. <//. ]>p. 'J-Jii-'J-j;;. On tin; embryonal byssus-gland of A noilnnhi, see 

 J. Carrierc, j/iiu/ui/. Inneiff. vii. |>. II. 



