MOLLUSCS. 309 



On a preceding page has been detailed the history of the liver-fluke, Distoma, which 

 passes a portion of its life in a species of Limnma, and this is far from a solitary- 

 example. These snails are eaten by fishes and birds, and in the stomach of the eater, 

 the larvsB are set free, and enter into a new stage of existence. In some few cases 

 the history has been thoroughly worked out, but in the majority there is a field for 

 investigation, which will give the careful student wonderful results. The subject is 

 difficult to study, but, with time and patience its j)roblems may be solved. 



Sub-Oedee II. — Stylommatophoka. 



The great majority of the Stylommatophora are terrestrial, and are readily distin- 

 guished from the other sub-order by having two pairs of tentacles, the superior pair 

 bearing the eyes at the extremity. These tentacles may be simply retracted, or, as in 

 the common snails the tip can be turned in like the finger of a glove, a condition de- 

 scribed as invaginate. In most of the sub-order the 

 genital orifices are united. 



The first family to be considered is the Onchidiidjb, 

 which embraces a few terrestrial and aquatic forms from 

 warm latitudes. They have the male and female orifices 

 widely separated, and the eye-bearing tentacles simply 

 contractile and not capable of being invaginated. No 

 shell occurs in the family. Onchidiwm has been ren- 

 dered prominent by the researches of Semper. This 

 naturalist studied one of the species found in the eastern 

 seas, and found that, besides the eyes borne on the end 

 of the tentacles, the whole dorsal surface was covered 

 with visual organs, a fact which will i-ecall the more re- 



=• . Fig. 360.— Onchidiwm, tongamim. 



cent discoveries of Moseley with regard to the chitons. 



These eyes are different from those borne on the tentacles in the fact that, in struc- 

 ture and development, they are like those of vertebrates, the nervous fibres penetra- 

 ting the layer of rods and cones, and being distributed over their inner surface. 



Whether these eyes exist in all species of Onchidium is not known, as all have not 

 yet been made the subject of proper histological investigation. Why these eyes 

 should be developed here is uncertain. The only explanation as yet advanced is that 

 of Semper. The Onchidia live on the shores of the ocean, where they creep about in 

 a slug-like manner. They play an important part in the diet of the jumping fish, 

 Periophthalmus, which leaves the water and travels about on the beach left bare by the 

 retreating tide, looking for food. Semper supposes that these eyes are of considerable 

 use in avoiding this enemy of the race. 



The genera OncMdella and Peronia are also marine and live on algae. They are 

 amphibious, and if kept moist, they can live for a long time removed from the water. 

 Veronicella is a terrestrial genus represented by a single species in Florida, and several 

 others in the tropics of the old and new worlds. They live in families under trees and 

 stones, whence they come forth at night, ascending trees, etc., in their search for food. 

 Unlike the slugs, which they resemble in general appearance, they leave no slimy tracks 

 behind them. They lay their eggs in long gelatinous threads, fifteen or twenty being 

 contained in a single string. 



In all the remaining families the generative orifices are united, but the question of 



