HARDWICKE'S SCIENCE-GOSSIP. 



195 



end consists of the numerous small clear prismatic 

 cells, which are now furnished with fiagella, projecting 

 from their outer ends; the larger end consists 

 of the fewer (thirty-two) larger, rounded, dark, 

 granular cells, sixteen of which are arranged to form 

 a girdle round the equator, next to 'the prismatic 

 half. The cleavage cavity still exists as a more 

 or less spherical space in the middle, bounded half 

 by the pigmented ends of the prismatic cells, half 

 by the granular cells. By the movements of its 

 l]agella the larva now is liberated from its encapsuled 

 cavity in the mesoderm, passes through the endo- 

 dermal layer, into the radial tube, and so borne 

 along by the outflowing currents of the circulation 

 out of the sponge into the surrounding water, where 

 it spins about in a lively whirling kind of dance. As 

 it grows, the little "blastula," as it may now be 

 called, becomes less ovate and more spherical, and 

 then commences to pass through one of the most 

 important stages of its existence ; the flagellated 

 layer begins to lose its spherical contour, becomes 

 flattened, depressed, and is at length drawn quite 

 within the hemisphere of granular cells ; it then 

 applies itself to the inner face of this layer, entirely 

 obliterates the cleavage cavity, and thus gives 

 rise to the true gastrula form. It will be seen 

 that the embryo is now somewhat beehive-shaped, 

 its wall is double, consisting of an outer ectodermic 

 layer of granular cells, and an inner endodermic 

 layer of prismatic cells, the cleavage cavity has 

 disappeared, and a new cavity with a widely-open 

 aperture below has arisen by invagination. This 

 aperture is bounded by the row of sixteen granular 

 cells, which previously formed the equatorial girdle 

 of the embryo, they now grow radially inwards 

 towards the centre of the aperture, and thus diminish 

 its area, till it becomes reduced to a comparatively 

 small opening ; it is the larval mouth, as the cavity 

 produced by invagination is the larval stomach. So 

 far the larva has led an active, if not an industrious, 

 existence, with the continual promise of better things ; 

 it now proceeds to a step, which, in the history of 

 animal development, has generally proved fatal to 

 further progress of importance, and has indeed often 

 led to change in a backward direction. Settling 

 down on some fixed object, such as a bit of stone, or 

 the stem of a seaweed, it exchanges a free for a fixed 

 and stationary existence. The granular cells sur- 

 rounding the mouth grow inwards towards its centre, 

 and completely obliterate it, at the same time they 

 grow outward over the surface of attachment in trans- 

 parent, irregular, jagged, pseudopodia-like processes, 

 which solder the young sponge securely to its seat 



(fig- 97)- 



By the absorption of a part of their granules, the 

 granular cells lose, to a great extent, their opacity, so 

 that one can see the layer of prismatic, or cylin- 

 drical cells, through them. They have, evidently, at 

 this stage, lost their fiagella. By excretion, or 



alteration, of the granular cells, a thin layer of jelly- 

 like material is formed between the two layers of the 

 embryo ; this is the beginning of the future mesoderm, 

 and in it the spicules as delicate needle-shaped forms 

 first make their appearance. As they grow they 

 soon enter the outer, or ectodermic, layer. 



The larva grows in the direction of its axis, and 

 so elongates into a cylindrical or conical body, the 

 distal end flattens, and becomes perforated by a hole 

 which puts the stomach cavity into communication 

 with the surrounding water. This hole is the 

 adult mouth, the endodermal cells retreat from it, 

 leaving the ectodermal layer alone around it, as a 

 clear thin membrane terminating the gastral cavity. 

 Little round spaces open in the side-walls of the 

 sponge and form the pores, the endodernjal cells 

 acquire their characteristic collar and re-acquire their 

 flagella. Simultaneously with these changes addi- 

 tional spicules appear. The double-pointed needle- 

 like spicules projecting obliquely upwards and 

 outwards, form a sort of tube, extending from the 

 base to the summit ; immediately round the basal 

 edge and the summital edge, they project outwards 

 at right angles, forming a kind of collar. Mingled 

 with the basal spicules are those with toothed ends, 

 before alluded to as grapnel-like spicules. Between 

 the needle-like spicules are tri- radiate ones, all simi- 

 larly arranged, two rays being directed more or less 

 concentrically, and the third longitudinally and 

 downwards. Quadriradiates also make their appear- 

 ance, and first round the terminal edge, two of their 

 rays lying in the edge, one being directed longitudi- 

 nally downwards, the other radiately towards the 

 mouth, aiad serving as a support for the oral or 

 terminal membrane. They are usually four, six, or 

 eight in number at first, and are always symmetrically 



arranged, /t^ / t- j\ 



'> {lo be contimtca.) 



RUSTIC BOTANY, 

 No. II, 



IN connection with a former paper, " On Rustic 

 Botany," we proceed to consider: Plants used 

 in domestic medicine, for their real or supposed 

 virtues, omitting such plants as enter into our materia 

 medica, and such as were formerly used, upon the 

 principle of "signatures," that is, the doctrine that 

 the medical virtues of plants are indicated by some 

 outward signature, or markings, colouring, &c. 



Parsley-piert {Alcheniilla arvensis) is a lowly little 

 plant, more admired for the pleasant green of its 

 foliage, than for the showiness of its golden green 

 flowers ; this is used by surgical magnates of no 

 insignificant authority. It is astringent, and perhaps 

 slightly mucilaginous, Alchemilla is from the 

 Ai'abic, Alkiindyek, alchemy ; on account of its 

 pretended alchemical virtues. (Theis.) The buck- 

 bean, or marsh trefoil {Meiiyanthes trifoUata) is a 

 beautiful plant, and good as a bitter. Linnseus 



K 2 



