222 



ELEMENTARY ZOOLOGY. 



244. Reproduction and Development. The reproduction is always 

 sexual, though the sexes are often in the same individual (as in the 

 oyster, and many of the gasteropods) . The eggs, after being 

 fertilized, are laid singly or in masses, often surrounded by gelatinous 

 secretion (as in the snails) which serves as a means of protection 

 from drouth, and for attachment. In cephalopods the eggs are 



FIG. 71. Diagram showing various types of invertebrate eyes. B, C, and D show 

 conditions in various mollusks. Eyespots similar to A and E are found in certain 

 jelly-fish, ep, epidermis; /, nerve fibre; /, lens; op, optic nerve; p, pigment cells; r, 

 retina; v.h., vitreous humor. F shows a few cells of retina much enlarged. 



Questions on the Figure. Begin with A and describe the progress- 

 ive changes that take place in this set of figures. Discuss the 

 possible value of each change. In what way would B be a better 

 eye than A? What two elements are shown in the retina? What is 

 the function of pigment ? 



large and contain a large amount of nourishment. The cephalopods 

 and the land snails do not undergo a metamorphosis. The other 

 types pass through larval stages similar in some respects to those of 

 the marine worms (trochosphere). In the clam the larva is parasitic 

 during a part of its life. 



245. Relation to Nature. The mollusks have been a 

 most successful group of animals from the first geological 



