522 MOLLUSCOUS ANIMALS GENERALLY. 



which are usually attached to sea-weeds or zoophytes. The 

 course of development, in the first and last of these instances, 

 may be readily observed from the very earliest period, down to 

 that of the emersion of the embryo; owing to the extreme 

 transparency of the "nidamentum," and of the egg-membranes 

 themselves. The first change which will be noticed by the ordi- 

 nary observer, is the " segmentation" of the yolk-mass, which 

 divides itself (after the manner of a cell undergoing duplicative 

 subdivision) into two parts, each of these two into two others, 

 and so on, until a mulberry-like mass of minute yolk segments 

 is evolved. Glenerally speaking, however, there may be noticed 

 at a very early stage of this process, as performed by Gasteropod 

 Mollusks, an inequality in the size of the segments (Fig. 271j 

 c) ; one set, derived from the larger of the two divisions _(d) into 

 which the yolk-sphere first separates itself, being destined to 

 form the internal organs, whilst the other set of segments, of 

 much inferior dimensions, and formed by the subdivision of the 

 smaller half of the original sphere, furnishes the material for 

 the superficial parts. Soon after the "mulberry mass" has been 

 formed, it commonly begins to exhibit a very curious alternating 

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

 direction, and the same number in a reverse direction: this 

 movement, which is due to ciliary action, is often extremely 

 transitory in its duration ; but in the Lymnceus it continues 

 almost up to the escape of the embryo, and, when several ova 

 are brought into view at once under a low magnifying power, 

 the spectacle is a very curious one. 



350. 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, consists in its extension into a sort 

 of fin-like membrane on either side, the edges of which are 

 fringed with long cilia (Fig. 270), whose movements may be 

 clearly distinguished whilst the embiyo is still shut up within 



the egg ; at a very early 

 ^^°- 2'^°' period may also be discern- 



ed the "auditory vesicles" 

 or rudimentary organs of 

 hearing (§ 353), which scarce- 

 ly attain any higher deve- 

 lopment in these creatures 

 during the whole of life ; 

 and from the immediate 

 ^ _ neighborhood of these is 



Embryoes of NudibrancMate Gasteropods. put forth a projectioil, Avllich 



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 



