MOLLUSCS. 



305 



forms by an upper jaw composed of either one or three pieces ; in others, no upper jaw 

 is present. 



The generative apparatus is rather complex and affords good systematic charactei-s. 

 The most noticeable feature is that from the ovo- 

 testis a single duct proceeds, which afterward 

 divides into two tubes, one connecting with 

 the male, the other with the female copulatory 

 organs. In the Helicidae peculiar crystalline, 

 fluted, chitinous, or calcified rods or darts {spic- 

 ula amoris) are formed, the functions of which 

 are still problematical. 



The eggs are laid in moist places, in damp 

 earth, under dead leaves, etc. ; or, by the aquatic 

 species, in the water. Those of Limnma and 

 Planorbis are easily studied during their devel- 

 opment. In Ziimax, — the eggs of which are 

 laid separately, each one resembling a drop of 

 dew — when the embryo is far along in its de- 

 velopment, a peculiar pulsating sac is formed 

 in the middle of the foot, the function of which 

 is as yet unknown. 



Sttb-Obdee I. — Basommatophoka. 



The position of the eyes affords a good char- j.^. 348. -Embryo of Umax; d, yolk; e, eye; 



acter for a division of the Pulmonata into two {{itl'^^lrL^k'^Tluin^^^'^n^i:^^ 

 sub-orders. In the present group the visual granules. 



organs are seated at the base of the solid, contractile feelers ; the velum of the larva is 

 retained in the adult ; and the male and female generative apertures are separate and 

 placed on the right side of the neck. Most of the members of the sub-order are aquatic 

 in habits, though some lead more or less terrestrial lives. 



The family Amphibolidje, which occurs only in the New Zealand seas, serves to 

 connect the pulmonates with the opisthobranchs. They live in the salt marshes, 

 where the water is at least brackish, but are partially aerial in their resjjiration, although 

 rudimentary gills are present. The shells are closed by a horny operculum. In 

 these two features the Amphibolidse differ from all other pulmonates. The shell is 

 spiral and thick, the spire short and the whorls shouldered. The native New Zea- 

 landers eat the animal. 



The families Gadinid^e and Siphonaeid^ embrace together about a hundred 

 species of limpet-like pulmonates, with shell and habits nearly like Acmoea and Patella. 

 No species are found on the east coast of the United States. 



Concerning the Aueiculid^ more can be said. The animals are mostly tropical; 

 still several small species are found even in the northern states. The spiral shell is us- 

 ually thick and solid, and covered with an epidermis. The spire is short, the body 

 whorl large, and the outer lip is thick and frequently armed interiorly with teeth 

 which considerably contract the aperture. Similar teeth are found on the columellar 

 lip. The respiratory pore is posterior, and the male and female reproductive organs are 

 widely separated. The mouth is armed with a horny jaw. 



VOL. I. —20 



