122 



HARDWICKE'S SCIENCE-GOSSIP. 



land, freshwater, or marine species, obtain their hnie 

 from the plants on which they feed. These have 

 previously extracted it from the water or the earth. 

 Nearly all sea-shells are thick and heavy, those of the 

 land neither so thick or so heavy, whilst freshwater 

 shells are very thin and very light. In the sea there 

 is an abundance of carbonate of lime held in solution, 

 and ocean-plants contain large quantities of it. 

 Marine molluscs subsist on these ocean-plants, and 

 consequently a large amount of lime enters their 

 system. This lime is secreted by the outer surface of 

 the mantle, and after it has been exposed to the air 

 for a short time it becomes hard. With respect to 

 land-snails, we find that in clay districts they are 

 either rare and thin-shelled, or absent altogether. In 

 chalky districts, on the contraiy, they are usually 

 abundant and the shells are relatively thicker. One 

 species, our largest native snail, is only found in the 

 chalk districts, and is especially plentiful on the North 

 Downs. This is the Roman snail, a species said to 

 have been introduced by Cocsar's legions on account 

 of its high character in the eyes of the Roman epicures. 

 The shell of this species is very thick and solid, in 

 comparison with that of other snails. From these 

 statements it is easy to understand why our pond- 

 snails are thin-shelled, for ponds with clay bottoms 

 contain but little carbonate of lime. But, in addition, 

 ponds of still water in which there is decaying vege- 

 tation and animal matter usually contain large 

 tiuantities of carbonic acid gas, which is very destruc- 

 tive to lime. Instances of this may be seen among 

 these pond-snails before us, for some of them have 

 the tip of the shell all eaten away, or eroded. This 

 is solely due to the action of carbonic acid gas upon 

 the lime. 



Jiut why is it only the fip of the shell that is thus 

 eroded ? Why does not this carbonic acid gas attack 

 the lower and wider coils of the shell ? The whole 

 shell is protected by a layer of animal matter — the 

 epidermis — which is endowed with life, and therefore 

 proof against the attacks of this acid. But at the tip 

 of the shell — which was formed by the snail when 

 very young, this epidermis is very thin, and conse- 

 quently more easily destroyed by the acid. Spiral 

 shells are frequently found with the first few whorls 

 completely missing, and in the flat discoidal trumpet 

 snails specimens may be seen with a hole through the 

 centre where the shell has been removed. 



Now let us take a glance at the history of our 

 water-snail. Every individual in itself combines the 

 functions of both sexes, and in summer they may be 

 seen depositing long curved cylindrical masses of a 

 clear jelly-like substance on the walls of their tank. 

 Should this be of glass we may examine them without 

 removal. Applying our lens to the glass we shall 

 observe that a great number of minute clear globules 

 are dotted throughout its substance. These globules 

 are none other than the eggs or ova of our water- 

 snail, each one containing a germ of life, destined to 



pass through a regular and well-marked course of 

 development until it arrives at full-grown snail-hood 

 like its parent. 



Let us place one of these spawn-masses upon a 

 glass slip and then on the stage of our microscope. 

 For such an object we require what is known as a 

 low-power objective, that is, one of the least magni- 

 fying power. We fix on the one-inch objective and 

 carefully focus a single ovum or egg. It is revealed 

 to us as a large oval vessel filled with a clear pris- 

 matic liquid, the albumen. At one portion of this 

 vessel, which we may term the egg-shell, is a denser 

 globular mass which is the yelk. Within this is a 

 part still more dense, the germinal vesicle, with a 

 clear central space or nucleus. This germinal vesicle 

 is a single cell, and from a similar single cell every 

 animal and every plant is developed. However 

 widely organic forms may differ in the adult state they 

 may all be traced back through a series of gradual 

 changes to a solitary little cell, scarcely differing 

 in appearance from this cell before us. The lowest 

 form of animal life with which we are acquainted 

 consists of one cell only and never gets beyond it. 

 So, too, the lowest forms of plants are unicellular. 

 It should also be borne in mind that all forms of life 

 are reproduced by the division of these cells. There 

 is no such phenomenon as spontaneous generation. 

 No living cell can be produced save by the division 

 of an already existing cell. 



Now we will have another look at our germinal 

 vesicle, or perhaps it will save time to speak of it as 

 the embryo. There is the embryo in the yelk sus- 

 pended in the albumen. Now let us leave it and 

 inspect it again in a few hours' time. We shall then, 

 find that the embryo is dividing, or has divided, into 

 two equal portions or cells each with the clean central 

 space. Next day we shall find that each of these 

 two cells have, in their turn, divided in like manner, 

 so that we have four cells contained in the enveloping 

 membrane of the yelk. Next we find a group of 

 eight cells, then sixteen, and so on, always increasing 

 in multiples of four until at length we have what 

 biologists term a " mulberry mass." At this stage the 

 cells are too numerous to be counted, and form a mass 

 much resembling a mulberry or blackberry. By-and- 

 by the yelk develops from its outer surface a band 

 of very delicate filaments, or cilia. By the lashing of 

 these cilia rotatory motion is imparted to the embryo 

 and it takes several turns to the right, and then 

 reverses the engine and makes as many revolutions to 

 the left. Henceforth there is continual motion in 

 our embryo, although as yet it is totally devoid of 

 organs. It is still a mere aggregation of cells, with- 

 out these cells being arranged into tissues or vessels. 

 But soon it shows a disposition to lengthen in one 

 direction and assumes a bi-lateral symmetr)', A little 

 later it shows indication of a division into anterior and 

 posterior portions. The large cells are divided and 

 broken up into very much smaller ones, and these 



