186 THE MICROSCOPE AND ITS REVELATIONS. 



two others, and so on until a morula or mulberry-like mass of minute 

 yolk-segments is produced (Fig. 403, A-F), which is converted by 'in- 

 vagination' into a 'gastrula' (391), whose form is shown at G. This 

 'gastrula' soon begins to exhibit a very curious alternating rotation 

 within the egg, two or three turns being made in one direction, and the 

 same number in a reverse direction : this movement is due to the cilia 

 fringing a sort of fold of the ectoderm termed the velum, which after- 

 wards usually gives origin to a pair of large ciliated lobes (H-L, c) resem- 

 bling those of Kotifers. The velum is so little developed in Limnceus, 

 however, that its existence has been commonly overlooked until recog- 

 nized by Prof. Kay Lankester/ who also has been able to distinguish its 

 fringe of minute cilia. This, however, has only a transitory existence; and 

 the later rotation of the embryo, which presents a very curious spectacle 

 when a number of ova are viewed at once under a low magnifying power, 

 is due to the action of the cilia fringing the head and foot. 



582. A separation is usually seen at an early period, between the 

 anterior or 'cephalic' portion, and the posterior or 'visceral' portion, of 

 the embryonic mass; and the development of the former advances with 

 the greater activity. One of the first changes which is seen in it, con- 

 sists in its extension into a sort of fin-like membrane on either side, the 

 edges of which are fringed with long cilia (Fig. 403, H-L, c), whose move- 

 ments may be clearly distinguished whilst the embryo is still shut-up 

 within the egg; at a very early period may also be discerned the 'auditory 

 vesicles ' (K, x) or rudimentary organs of hearing ( 587), which scarcely 

 attain any higher development in these creatures during the whole of 

 life; and from the immediate neighborhood of these is put-forth a pro- 

 jection, which is afterwards to be evolved into the 'foot' or muscular 

 disk of the animal. While these organs are making their appearance, 

 the shell is being formed on the surface of the posterior portion, appear- 

 ing first as a thin covering over its hinder part, and gradually extending 

 itself until it becomes large enough to inclose the embryo completely, 

 when this contracts itself. The ciliated lobes are best seen in the embryoes 

 of Nudibranchs; and the fact of the universal presence of a shell in the 

 embryoes of that group is of peculiar interest, as it is destined to be cast- 

 off very soon after they enter upon active life. These embryoes may be 

 seen to move-about as freely as the narrowness of their prison permits, 

 for some time previous to their emersion; and when set free by the rup- 

 ture of the egg-cases, they swim forth with great activity by the action 

 of their ciliated lobes, these, like the 'wheels' of Kotifera, serving also 

 to bring food to the mouth, which is at that time unprovided with the 

 reducing apparatus subsequently found in it. The same is true of the 

 embryo of Lymnceus, save that its swimming movements are less active, 

 in consequence of the non -development of the ciliated lobes; and the 

 currents produced by the cilia that fringe the head and the orifice of the 

 respiratory sac, seem to have reference chiefly to the provision of sup- 

 plies of food, and of aerated water for respiration. The disappearance 

 of the cilia has been observed by Mr. Hogg to be coincident with the 

 development of the teeth to a degree sufficient to enable the young water- 

 snail to crop its vegetable food; and he has further ascertained that if the 

 growing animal be kept in fresh water alone for some time, without vege- 



1 See his valuable * Observations on the Development of Limnceus stagnalus, 

 and on the other stages of other Mollusca,' in " Quart. ^Journ. Microsc. Science," 

 Oct. 1874. See also Lereboullet, ' Recherches sur le Developpement du Limnee,' 

 in " Ann. des Sci. Nat. Zool.," 4ieme Ser., Tom. xviii., p. 47. 



