DEVELOPMENTAL HISTORY OF THE MOLLUSCA. 27 
sharply defined. In Plate 8. fig. 28 it has become fully differentiated from the overlying 
tissue, and consists of a separate pair of rounded bodies (of which one only is seen in 
this side view). In close connexion with it are other smaller rounded masses of the 
same appearance (ng' and ng"). It seems very probable that these are outgrowths from 
the primary nervous mass to form the pedal ganglia. ‘The otocyst is seen in close rela- 
tion with these supposed nervous masses. Of the otocyst it is merely necessary to point 
out that the cells surrounding it gradually form for it a definite wall, and that then in 
its centre appears a small otolith which gradually increases in size. It is not uncommon 
for the otolith to make its appearance in one of the two otocysts before it does in the 
other, as in Plate 10. fig. 5. 
The muscle of the yvelum marked mv in Plate 8. fig. 22 is worth mention, since it 
appears at an early period. It passes from the border of the velum to the foot. By 
the contraction of this muscle the velum becomes doubled to some extent on itself, as 
seen in figs. 30, 31, and the movement of the cilia stops. 
The sudden stoppage of the cilia of the velum during life, and the erect sheaf-like 
appearance which they assume, is quite different to the stoppage and disordered entan- 
glement which they exhibit when the embryo is killed by acid. The rigid character of 
the position of rest of these large cilia is exactly repeated in the case of the perianal 
-circlet of large cilia in such Annelidan embryos as that of Terebella. 
The first trace of the great posteriorly placed retractor muscle may be made out in 
embryos which are looked at from behind, when of about the same age as that of fig. 28. 
The further differentiation of this finely fibrillar muscular band is seen in figs. 31 & 34, 
M, M'. Iwas unable to observe the mode of development of this structure, though 
in some Nudibranchs its differentiation from corpuscular elements lying beneath the 
epiblast, and derived originally from it as a part of the parietal layer of the mesoblast, 
is clear enough. 
The matters of interest to which it has been the object of this part of the present 
communication to draw attention are as follows :— 
1. The primitive arrangement of the results of the cleavage-process. 
2. The mode of development of the otocysts, by vacuolation of the epiblast. 
3. The development of the cephalic-nerve ganglion-pair as a thickening of the 
epiblast. 
4, The “shell-patch,” “ shell-groove,” and its plug. 
5. Artificially produced monstrosities of the embryo. 
6. Points of wide divergence in the development of the alimentary tract, and its rela- 
tion to the yellow residual yelk-masses, between the two closely allied species here 
spoken of as Aplysia major and Aplysia minor. 
E2 
