MOLLUSCA. 249 



Though originating in a different way, it is strikingly like 

 the vertebrate eye. 



290. Library Reference. Make a report on the position and general 

 structure of the eyes in gasteropods, cephalopods and lamellibranchs. 



291. Reproduction and the Genital Organs. Reproduc- 

 tion is always sexual. In some of the lamellibranchs (e. g. f 

 oyster) and many of the simpler gasteropods, including the 

 land snails, the individuals are hermaphrodite. The sexes are 

 separate in the cephalopods and in most of the lamellibranchs 

 and gasteropods. The organs are more complicated among 

 the hermaphrodite gasteropods than elsewhere in the group 

 (see diagram reproductive organs of snail, Fig. 112). The 

 sexual glands are usually situated in the visceral mass among 

 the coils of the intestine. The ducts ordinarily open into the 

 mantle cavity where fertilization may occur. The eggs after 

 fertilization are often, either singly or in masses, surrounded 

 by a gelatinous secretion (as in the snail) which serves as a 

 protection from drouth and as a means of attachment. In 

 lamellibranchs the young are not infrequently retained in the 

 mantle or respiratory chamber until partly developed. 



292. Development. Segmentation is total (lamellibranchs 

 and gasteropods) or partial and discoklal (dibranch cephalo- 

 pods). It is usually unequal in the lamellibranchs and gastero- 

 pods, but in some of the latter it is equal during the first two 

 divisions, producing four equal blastomeres. Each of these 

 divides into a large and a small cell macromere and micro- 

 mere. Still other micromeres are formed at the expense of 

 the macromeres, and these by continued division form a cap 

 of ectodermal cells (Fig. 113). From the macromeres arise 

 ultimately the entoderm and mesoderm. The gastrula may 

 be formed either by invagination of the large cells or by the 

 overgrowth of the micromeres, depending on the size of the 

 segmentation cavity and of the entodermal cells. In the 

 cephalopods, owing to the large supply of food substance in 

 the ovum, cleavage is confined to a small disc at the active pole. 



