Ova of the Nudibranchiate MoUusca. 383 



days later. The case-membrane previous to the escape of the em- 

 bryo becomes gradually thinner, and at last either entirely disap- 

 pears or is reduced to shreds. This change in the case-membrane 

 may probably be in some measure caused by the incessant strokes 

 of the long cilia of the ciliated discs upon its inner surface during 

 the active revolutions of the embryo round its interior. The em- 

 bryo at the time of its liberation is provided with a shell (figs. 25 

 and 20 a), considerably longer in its antero-posterior than in its 

 transverse diameter, from which it can protrude the upper part 

 of its body and retract it at pleasure. Fig. 20 is a representation 

 of the embryo when protruded from its shell, and fig. 21 when en- 

 tirely drawn into its interior. The embryo with its shell is at this 

 period considerably larger than the ovum at the time of its extru- 

 sion. Some of them measured about l-145th of an inch in length 

 and 1 -200th in the antero-posterior diameter; others 1-1 70th in 

 the former and 1 -250th in the latter direction. The parts which 

 canbe protruded from the shell are two large andprominent ciliated 

 discs (fig. 20 c), and a projecting process* attached to the upper 

 part of the anterior surface of the body (fig. 20 d) . The lower 

 surface of this projecting process or foot is covered by a hard 

 plate (fig. 20^), which closely adheres to it and moves along with 

 it. The ciliated discs are higher in front than behind, and are 

 separated in front by a deep notch and by a shallower one behind. 

 They are very contractile, and present very difi*erent appearances 

 at different times ; and their superior surface is provided with a 

 thickened margin, to the upper and outer edge of which a row of 

 long and strong cilia is attached, by whose movements they can 

 swim rapidly in various directions through the water. When 

 these discs are elevated and in the vertical position, as repre- 

 sented in figs. 20 and 18, the parts connecting their margins to 

 the body of the embryo are translucent, and they may now be 

 contracted from before backwards and a number of the central 

 cilia collected into a tuft ; or if the embryo be about to retire 

 into the shell, they are pressed together, the translucent tex- 

 ture connecting their thickened margins to the body contracts 

 and pulls them downwards, and the foot with its hard plate is 

 raised, as is represented in fig. 21. This plate now acts as an im- 

 perfect operculum. When the embryo, on the other hand, is 

 about to swim, the ciliated discs are thrown apart and flattened, 

 as is represented in figs. 16 and 19 ; and in this position each disc 

 approaches the circular form, is hollow on the upper surface, and 

 their thickened margins are prolonged inwards along the edges 

 of the anterior notch, at the bottom of which they are continuous. 



* To an exactly similar structure in the embryo of the Asplysia Van Be- 

 neden (Annalos des Sciences NaUirelles, torn. xv. p. 123, 1841) has given the 

 name of foot, and we have here retained the appellation. 



