MOLLUSC A 253 



Though originating in a different way, these are strikingly like 

 the vertebrate eye. 



The forms with shells are not so responsive to stimuli as 

 free types such as cephalopods, but even the snails, and in less 

 degree the clams, are sensitive to light, contact, gravity, 

 chemical states of the air and water that would in us arouse the 

 sense of taste or smell. The most characteristic reaction of the 

 lower forms is to withdraw into the shell or to close it. Gravity 

 and light and odor also direct their motions. 



296. Library Reference. Make a report on the position and general structure 

 of the eyes in gasteropods, cephalopods and lamellibranchs. 



297. Reproduction and the Genital Organs. Reproduction 

 is always sexual. In some of the lamellibranchs (e.g., 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 gastero- 

 pods. The organs are more complicated among the her- 

 maphrodite gasteropods than elsewhere in the group (see diagram, 

 reproductive organs of snail, Fig. 114). 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. 



298. Development. Segmentation is total (lamellibranchs 

 and gasteropods) or partial anddiscoidal (dibranch cephalopods). 

 It is usually unequal in the lamellibranchs and gasteropods, 

 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 micromere. Still other 

 micromeres are formed at the expense of the macromeres, 

 and these by continued division form a cap of ectodermal cells 

 (Fig. 115). From the macromeres arise ultimately the ento- 

 derm and mesoderm. The gastrula may be formed either by 



