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- 

 pods 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 larvas on 

 their first emersion frpm 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 Cephalopods, 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 



