STRUCTURE OF THE SNAIL. 327 



Cupid's Dart-Sac, and contains a pointed calcareous arrow 

 (spiculum amoris), which is jerked out as a preliminary 

 excitant to copulation. The dart is sometimes found ad- 

 hering to the skin of a snail, and after copulation the sac is 

 empty — soon however to be refilled. 



When two snails pair, the genital apertures are dilated, 

 the protruded penis of one is inserted into the aperture of 

 the other, and the transference of a spermatophore is thus 

 effected. 



The eggs are laid in the earth in June and July. Each 

 measures about a quarter of an inch in length, and is 

 surrounded by gelatinous material acquired in the oviduct, 

 and by an elastic but calcareous shell. 



Segmentation is total but slightly unequal. As the snail 

 is a terrestrial Gasteropod there is no trochosphere larva nor 

 more than a slight hint of the characteristic MoUuscan 

 velum. A miniature adult is hatched in about three weeks. 

 The study of development may be more profitably attempted 

 in the pond snail LimncBus, where gastrula, trochosphere, 

 and veliger can be readily seen. 



Classification of Gasteropoda. 



Sub-Class I. Gasteropoda Isopleura (= Amphineura). 



Bilaterally symmetrical Gasteropods, elongated in form, with mouth 

 kt one end of the body and anus at the other. 



The pedal and visceral nerve-cords run straight, parallel to one another, 

 along the whole body. Ganglionic swellings are slight or even 

 absent. 



The auricles, the gills, the nephridia, the genital ducts, are paired 

 and bilaterally symmetrical. 



Examples. — Chiton, etc. (Polyplacophora), with eight dorsal shell 

 plates, each developed in a shell-sac comparable to the single one of 

 other Molluscs. Lodged in the shell there are sometimes hundreds of 

 eyes, but there are none on the head. The mantle flap which hangs 

 down the sides, often bears spines and knobs, some of which may be 

 sensory. Sheltered by the mantle, along the sides of the body are 

 many gills, each with "smelling patch," or " osphradium," at its base. 

 The odontophore is well-developed. 



Some small species are common on the rocks of British coasts. 

 Neomenia and Proneomenia are small, flat forms, whose shell is repre- 

 sented solely by microscopic plates or spicules of lime. The mantle is 



