DEVELOPMENT OF MOLLUSCA. 
577 
their adult condition, are adapted to move actively through 
the water : those of the Polyzoa swimming forth as ciliated 
gemmules; those of the Tunicata having a tadpole-like tail, 
formed by an outgrowth from the “ mulberry mass,” which 
propels them by its lateral strokes ; and those of the Gastero¬ 
poda having two large ciliated lobes, much resembling the 
“wheels” of Eotifera, placed one on either side of the mouth. 
It is further remarkable that all Gasteropods possess a shell 
in their early condition, whether or not they are to possess 
one ultimately. Among some of the Pectinibranchiata, which 
constitute the highest order of this class (Zool. § 985), a very 
remarkable provision has been observed for carrying-on the 
development of the embryo to a more advanced stage than is 
attained in other instances within the egg. In addition to the 
“germ-yolk,” which- undergoes the usual processes of fission 
and of conversion into the mulberry-mass (§736), by the sub¬ 
sequent metamorphoses of which the body of the embryo 
is formed, we find a quantity of “ food-yolk ” stored-up with 
each embryo, which may be likened to the supply that is 
provided by many Insects for the nutrition of their larvae on 
their first emersion from the egg (§ 703); this store is greedily 
devoured by the embryos, as soon as they have a mouth to 
swallow it and a stomach to hold it; and it is at the expense 
of this, that all their later development is carried on. 
753. The class of Cephalopoda, in which the sexes are 
always separate, presents us with some extremely curious 
provisions for bringing the products of the sperm-cells into 
contact with the eggs. "Whilst passing through the duct that 
conveys them forth from the glandular organ within which 
they are formed, the spermatozoids cluster together; and these 
clusters become invested with peculiar casings, which, when 
immersed in water, have a peculiar movement that enables 
them to advance through it, and causes them, when they meet 
with an obstacle, to rupture and set free their contents. In 
this mode it is that the spermatozoids find their way into the 
midst of the large grape-like clusters of eggs which have been 
deposited by the females, and which thus receive the fertili¬ 
zing influence of the male.—In the Argonaut or “paper-nau¬ 
tilus ” (Zool. § 962) there is a still more remarkable provision 
for the same end. All the individuals of this species that 
form the beautiful paper-like shell from which it derives 
p p. 
