184: 



THE MICROSCOPE AND ITS REVELATIONS. 



lying coiled-up between the lateral lobes. The hollow of each valve is 

 filled with a soft granular-looking msss, in which are to be distinguished 

 what are perhaps the rudiments of the branchiae and of oral tentacles; 

 but their nature can only be certainly determined by further observation, 

 which is rendered difficult by the opacity of the valves. By keeping a 

 supply of Fish, however, with these embryoes attached, the entire history 

 of the development of the fresh-water Mussel may be worked out. 1 



581. In certain members of the Class Gasteropods, 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 t water-snail ' of our ponds 



FIG. 401. 



FIG. 402. 



Palate of Buccinum undatum as 

 seen under Polarized Light. 



Parasitic Larva (Glochidium) of Anodon: A, glochidia at- 

 tached to the tail of a Stickleback ; B, side view of glochidium 

 still inclosed in the egg-membrane, showing the hooks of its 

 valves and the byssus-filamenta, 1 c, giochidium with its valves 

 widely opened, showing the adductor-muscle a; D, side view 

 of glochidium; with the valves opened to show the origin of 

 the byssus-filament and the three pairs of tentacular ( ?) or- 

 gans, the barbed hooks 6, and the muscular or membran- 

 ous folds c, c, connected with them. 



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

 a sixpence, in which from 50 to 60 eggs are imbedded, and which is at- 

 tached to the leaves or stems of aquatic plants; in the Buccinum unda- 

 tum, or common Whelk, it is a membranous case, connected with a con- 

 siderable 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 Purpura 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 the tide-marks, great 

 numbers being often found standing erect side by side; whilst in the 



l See the Rev. W. Houghton ' On the Parasitic Nature of the Fry of the Ano- 

 donta cygnea,' in "Quart. Journ. of Microsc. Sci.," N.S., Vol. ii. (1861), p. 162; 

 and Balfour, op. cit., pp. 220-223. 



