Molluscs. 



29 



breathe mainly through the general surface of the boch?, althougli 

 the large papillose appendages disposed in rows on the back 

 (simple in Acolis, branched in Triton ia) ma}- have some 

 respiratory function. The British species of Acolis, of which 

 there are many, A. papulosa being the largest, are often found 

 among rocks crawling over colonies of zoophytes, which they 

 devour. The eggs of our Sea-slugs are laid in gelatinous bands, 

 and the fry that issue from them are provided with a thin, 

 transparent shell, closed bj? an operculum, which is cast off 

 in the course of development. The bands of eggs may be so 

 long and convoluted as to have gi\'en rise to the name " Vermi- 

 celli di mare," by which the spawn of the Sea-hares is known to 

 Italian fishermen. The spa\ra of a single 

 Doris ma}' contain six hundred thousand 

 eggs. Unlike the Sea-snails, in which 

 the sexes are always distinct, but like the 

 land and fresh-water snails and slugs, 

 the Sea-hare and the Sea-slugs are 

 hermaphrodite. 



The Chitons, or coat-of-mail shells, 

 are often placed with the Gastropods iji 

 classification. TheA' differ, however, from 

 these as well as from all other molluscs in 

 having the shell composed of eight pieces 

 overlapping like the tiles of a roof. They 

 are s^'mmetrical, and the animal has 

 rather the shape of a limpet, the foot 

 being flat and adapted for crawling over 

 rocks and stones. The}' have the power 

 of rolling themselves up after the manner 

 of wood-lice. The sexes are distinct. 

 Nearly all the British species are small, 

 fascicidaris) not exceedin_ 



The Pelecj'pods are Molluscs essentialh' modified ivr a 

 sluggish existence, the t\'pical members i>f tlie group being 

 more or less sedentary and feeding on the mmute vegetable 

 and animal organisms in suspensioi in the water. They 

 possess neither a distinct head nor cephahc sense-organs, 

 and the whole body is enclosed in two large mantle folds, which 

 secrete a shell consisting of two valves. The gills are usualh' 

 large and lamellar or plate-like, hence the term Lamellibranchs 

 often applied to the group, also known as Bivalves. The name 

 Pelecypods alludes to the fact that the foot is often hatchet- 

 shaped, and thus adapted for ploughing through sand and soft 



FIG. 28.— CHITON 

 1■.^SCICULARIS. 



tlie largest (Chiton 

 a length of t\\'o inches (Fig, 28). 



