MOLLUSCS. 



347 



general shape is shown in the figure ; it is somewhat like a saucer without a bottom, 

 but the two ends of the ribbon are not united. On holding one uj) to the light, the 

 numerous eggs are seen as light spots arranged in quincunx. For a long time these 

 sand-saucers were a puzzle to naturalists, and under several diiJerent names they were 

 regarded as Polyzoa. The peculiar shape of the ribbon is due to the method of its 



Fig. 449. — Santi-saucer, egg mass of Naiica her 



formation. As it passes out of the parent it passes over the foot and is pressed against 

 the body of the shell. At first it is soft, gelatinous, and adhesive, but soon it hardens 

 and unites firmly to the surface some of the surrounding sand. Mr. A. H. Tuttle has 

 recently studied the develojnnent of these eggs. 



Tiie shell of Sigaretits is much like that of Haliotis, but lacks the series of perfora- 

 tions characteristic of that form. The species 

 frequent sand flats in the warmer waters of the 

 world, where they crawl in a sluggish manner 

 searching for food. They are very timorous 

 and retract themselves on the slightest alarm ; 

 but, owing to the large size of the aperture and 

 the miinite operculum, they are not much better 

 off after having withdrawn themselves as much 

 as possible into the shell. Lamellarla, in shape 

 of shell, holds a position about midway between 

 Ncitica and Sigaretus, but has no operculum. 

 Only ten species are known, none of them from 

 America. The following observations apjily to 

 L. 2)ers2ncua, a European sjsecies. The time of 

 s])awning is during February and March. At 



that time the female eats a round hole in the colonies of one or more species of com- 

 ])ound ascidians, this it lines with a pot-shaped capsule furnished with a transparent 

 lid. Ill this the eggs are placed, and, as they increase in size, the whole nest rises up 

 beyond the surrounding portion of the ascidian. At last the embryos are ready to 

 begin their free life, and then the lid bursts open and out come the young. By some 

 the genus Entoconclia is placed in or near this family. 



With the family Cerithiid^e we return to a group of molluscs with long and 

 slender shells, resembling in many respects the Molanians, but in others departing 

 considerably from them. The shell is long and many-whorlod, the aperture with an 

 anterior canal and a second less distinct posteriorly. The proper habitat of the 

 family is in the tropics, though numerous members arc found in colder waters. Some 

 of the species are marine, some live in brackish water, while others inhabit streams 



FlO. 450. — I^atica duplicata. 



