936 MOLLUSCA AND BEACHIOPODA 



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 ecto- 

 derm termed the velum, which afterwards usually gives origin to a 

 pair of large ciliated lobes (H-L, c) resembling those of Rotifers. 

 The velum is so little developed in Limnceus, however, that its 

 existence was commonly overlooked until recognised by Professor 

 Ray Lankester, 1 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. 



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 are seen in 

 it consists in its extension into a sort of fin-like membrane on either 

 side, the edges of which are fringed with long cilia (fig. 711, H-L, c), 

 whose movements may be clearly distinguished whilst the embryo is 

 still shut up within the egg ; at a very early period may also be dis- 

 cerned the ' auditory vesicles' (K, x) or rudimentary organs of hearing, 

 which scarcely attain any higher development in these creatures 

 during the whole of life ; and from the immediate neighbourhood of 

 these is put forth a projection, which is afterwards to be evolved into 

 the ' foot ' or muscular disc of the animal. While these organs are 

 making their appearance, the shell is being formed on the surface of 

 the posterior portion, appearing 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 embryos of Nudibranchs ; and the 

 fact of the universal presence of a shell in the embryos of that group 

 is of peculiar interest, as it is destined to be cast off very soon after 

 they enter upon active life. These embryos 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 rupture 

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

 of their ciliated lobes these, like the 'wheels' of Rotif era, 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 supplies of food and of aerated water for respira- 



1 See his valuable ' Observations on the Development of Limnceits stagnalis and 

 on the early stages of other Mollusca ' in Quart. Journ. -Microsc. Sci. October 1874 ; 

 and ' On the Developmental History of the Mollusca,' Phil. Trans. 1875. See also 

 Lereboullet, ' Recherches sur le Developpement du Limne'e,' in Ann. des Sci. Nat. 

 Zool. 4 e se'rie, torn, xviii. p. 47. 



