1 86 ELEMENTARY BIOLOGY. [CHAP. 



and the granular layers. These structures are properly 

 modifications of the epidermis; inasmuch as the cerebral 

 vesicle, of which the retinal elements are outgrowths, is an 

 involution of the epidermis of the embryo, and, morpholo- 

 gically speaking, the free ends of the rods and cones of the 

 vertebrate eye are, as in the crustacean, turned outwards. 

 There is good reason for believing the crystalline cones to 

 be derivatives of the investing epidermis. 



The auditory organ of the Lobster and Crayfish is situated 

 in the basal joint of the antennule, on the dorsal surface of 

 which its small slit-like opening, protected by numerous seta?, 

 is to be seen. The chitinous layer of the integument is 

 invaginated at the opening, and thus gives rise to a small 

 flattened sac lodged in the interior of the antennule. One 

 side of this sac is in-folded so as to produce a ridge, which 

 projects into the cavity of the sac, and is beset with very fine 

 and delicate hair-like setae. The auditory nerve enters the 

 fold, and its ultimate filaments pass into the setae at their 

 bases. The sac contains water in which minute particles of 

 sand are suspended in the manner of otoliths. 



The sexes are distinct in the Lobster and Crayfish. The 

 external characters of the males and females and the form 

 of the reproductive organs are described in detail in the 

 Laboratory work. 



The ovary is median and saccular, and its investing mem- 

 brane is prolonged backwards to form a paired oviduct 

 whose walls are glandular. Each ovicell is invested, during 

 its maturation, in an epitheloid follicle of a single layer of 

 cells; by the rupture of this the ripe ovum is liberated, and 

 thrown off thus into the interior of the ovary it makes its 

 way down the oviduct and so to the exterior. The impreg- 

 nated ova are attached in great numbers, to the hairs of the 

 swimmerets where they undergo their development, by the 

 viscid secretion of a scries of "cement glands" borne upon 



