DEVELOPMENTAL HISTORY OE THE MOLLUSCA. 
13 
No. II. The Early Development of two Species of Aplysia (Aplysia depilans and 
Pleurobranchidium, sp.). 
At Naples, in the winter of 1871-72, I searched for the ova of some Gasteropodous 
mollusk which would by their transparency permit the same kind of study with high 
powers as to the early phenomena of development as those of Pisidium had previously 
enabled me to carry on. Generally the ova of Mollusca are so highly charged with 
finely granular matter, and the limits of the individual embryonic cells so little defined, 
that it is impossible to do much with them on account either of opacity or of in- 
definiteness. The eggs of some Nudibranchs afforded interesting results as to the mode 
of formation of the “ Gastrula ” by invagination, which form the subject of a further 
communication ; but the particular ova which seemed most favourable for study, on 
account of transparency, clean definition of parts, and unlimited abundance, were those 
of Aplysia. I kept the eggs of two species of this genus (or rather species of Aplysia 
and of the subgenus Pleurobranchidium) under examination from time to time during 
several months. The eggs occur in masses, which resemble vermicelli, and are known 
by that name to the Neapolitan fishermen. The object of my work did not lead me to 
identify the precise species of Aplysia to which my observations refer. I am, however, 
able to identify the egg-coils ; and it is sufficient for all questions of histological and 
embryological interest to distinguish these as the larger and the smaller species of 
Aplysia (A. major and A. minor). I am nearly certain that my A. major is the common 
big A. depilans. It is the largest Aplysia which is common in the Bay of Naples. On 
the other hand, all I can say of my A. minor is that it is a much smaller species than 
the former ; and from comparison of eggs laid by a Pleurobranchidium , I take it to be a 
species of that subgenus. The egg-coils are distinguished by their size. Those of 
A. major are about one tenth of an inch in diameter, whilst those of A. minor are but 
two thirds of that width. The coils are, further, very completely distinguished by struc- 
ture involving a numerical character. The substance of the coils is a crisp gelatinous 
material, in which are closely packed spherical capsules (Plate 5. fig. 1, a). These 
capsules are of nearly the same size in the two species — a very little larger in the larger 
species. But whereas in the larger species each capsule contains from thirty to forty 
ova, each one of which undergoes development up to a far-advanced stage, in the 
smaller species each capsule contains but from five to seven ova, each one of which 
develops and finally emerges from the capsule as a swimming embryo. 
In the case of the smaller species, I kept the eggs from the earliest condition of 
cleavage to the liberation of the veliger embryos ; but when once free I could no longer 
retain them in my tank, since they were carried away by the stream of sea-water which 
it was necessary to use to ensure aeration. The constant injection of a fine jet of air 
into a small vessel of sea-water might obviate the difficulty which the water-stream 
always presents in the treatment of minute swimming embryos. 
In the case of the larger species, I never actually hatched any of the embryos, though 
the condition of Plate 6. fig. 37 cannot be far from that in which the embryo escapes. 
