1 86 ELEMENTARY BIOLOGY. [CHAP. 



rods and cones arise as outgrowths of the optic cup, which 

 is a derivative of the invaginate brain (cf. ante, p. 172); 

 morphologically speaking therefore, 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 setae, 

 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. One pair of genital ducts is normally 

 present in each sex, but marked indications of their dupli- 

 cation occasionally occur (cf. pp. 191 and 218). 



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 series of " cement glands" borne upon 



