64 MOLLUSCA. 
roo rows of teeth (Fig. 67). Some bore into shells, and 
all are scavengers. 
VALUE.—Many of the whelks afford a rich crimson dye, which in 
1684 was used in Ireland for dyeing linen. 
Note.—Some of the Southern species, as AM/agilus, burrow into 
coral, throwing out a tunnel as fast as the coral surrounds them, and 
filling it up with shelly matter. 
Diverse-footed Mollusks (eteropoda).—These Gas- 
teropods are all marine, and float upon the sea in trans- 
parent shells of delicate and glass-like construction. In 
some the foot forms in part a curious fin-like organ. The 
eggs are deposited in long threads, the young passing 
through many changes. The Carinaria (Fig. 65, C) and 
Atalanta are the best known. 
Gasteropods with Exposed Gills (Opisthobranchi- 
aia).—We now come to the shelled or shell-less mollusks 
‘that have gills more or less upon the outside. 
Sea-Pigeons (4p/ysiadz).—These are also called sea- 
slugs and hares. One, common on the Florida reef, is 
as large as the closed fist, and somewhat resembles a 
plucked pigeon. They are green or olive in color, and 
when touched throw out as a defense a cloud of rich pur- 
ple ink that completely surrounds them. They are found 
crawling among the sea-weed, not in the coral. The skin 
appears to cover the body in two folds lapping over the 
back, and when the animal is lifted from the bottom is 
vigorously flapped. 
Sea-Slugss (Zo/is, Doris, etc.) —These curiously formed 
mollusks are found on sea-weed and in other localities. In 
many the gills resemble plumes and leaves, so that the ani- 
mals are almost perfect mimics of the weed, even in color. 
Such is the luminous Scyllea pelagica. The gills of the 
Doris (Fig. 68, 7) resemble a plant; the Eolis (Fig. 68, ¢) 
seems covered with grass ; while the Glaucus resembles a 
lizard:with three branching feet on each side. They are 
