MOLLUSCA 



241 



But the largest known species is found in Africa living on trees, 

 and having a shell twenty-five centimeters long ; the eggs of this 

 species are very large, and, as in some other large species, are 

 laid in calcareous shells. 



In the slugs (Fig. 248) the shell is either very rudimentary or 

 entirely absent. The mantle is present, however, like a cap on 

 the dorsal side of the body, 

 and at one side, usually the 

 right, is the respiratory 

 pore. The head bears two 

 pairs of tentacles, and eyes 

 as in the land snails. The 

 largest species in the 

 United States measures 

 about ten centimeters, but 

 in other countries there 

 are larger species. They 

 are chiefly nocturnal in 

 habit, and feed upon vegetables. In Europe they have become on 

 this account veritable pests, but in America they do no serious 

 damage. As they crawl along, a gland at the anterior end of 

 the foot emits a secretion of mucus, thus making a smooth road 

 for the foot ; and some slugs, after ascending trees, fasten this 

 mucus to a branch, and then let themselves down by stretching 

 it out into a thread, very much as a spider descends. 



Fig. 248. Umax, a slug. PO, pulmonary aper- 

 ture. (From the Cambridge Naturai History.) 



CLASS III. SCAPHOPODA 



The Scaphopoda (Gr. aKci^of, digging, and ttov<;, foot), a 

 small group consisting of the elephant-tooth shells, are impor- 

 tant because forming a sort of 

 connecting link between the pre- 

 ceding class and the succeeding. 

 They are marine Mollusca, living 

 in the sand or mud at depths of 

 from twenty to two hundred 

 meters, with slightly curved, tubular shell (Fig. 249), open at 

 both ends and larger at the ventral than at the dorsal end. The 

 animal itself is strictly bilaterally symmetrical, and is provided 



FlG. 249. Dcntalium . Outline of shell 

 showing general shape and the larger 

 circular opening. (Drawn from speci- 

 men.) 



