OF MOLLUSCA. 177 



Fam. XV. CHITONIDiE. 



Shells of 8 valves, forming an imbricate series on the middle of 

 the back. Valves with a more or less wide plate of insertion sunken 

 into the mantle, with a deep lateral notch on each side ; the ante- 

 rior and posterior valves often with other notches on the terminal 

 edges. Gills conic, lamellar, in two series, one on each side the 

 hinder part of the body. 



Many anatomists have proposed to separate these animals from the 

 Mollusca, by forming them into a distinct subclass, and have con- 

 sidered them more nearly allied to the Annelides ! ; but I cannot see 

 the slightest grounds for such a separation ; and the observation of 

 the development from the eggs by Loven has entirely disproved any 

 affinity to them. They chiefly differ from other Mollusca hi the 

 shells not being developed on the embryo until some time after they 

 are hatched. 



The eggs are loosely united in clusters, on stones. Each egg is 

 enclosed in a thick, vesicular, folded envelope. The embryo in 

 the egg is oval (fig. 95 «), without any trace of shell, divided by a 

 circular indentation into two nearly equal parts, close to which are 

 attached the cirri, by means of which the movements of the embryo 

 are effected. In the middle of the upper part there is a tuft of 

 very fine filaments which scarcely exhibit any movement. The 

 lower half has two dark points (eyes), one on each side close to the 

 indentation. 



The young, when hatched, swim round the cluster of eggs, and 

 are more elongated ; the front part is finely ciliated, and the anterior 

 tuft of filaments occasionally vibrates ; the hinder part extends more 

 rapidly, and becomes conic (figs, b & c). The back is marked with 

 seven cross furrows ; between these the first rudiments of the shell 

 make their appearance in the form of close granulations. The animal 

 bends frequently, is quite soft, can only swim, but soon after this 

 begins to crawl (fig. c), and by a circular indentation the mantle is 

 separated from the foot. The eyes are seen more distinctly to be 

 situated on the ventral side, and are indistinctly visible from above 



(fig.rf). 



The front portion of the animal is sprinkled with acute tubercles, 

 principally on the upper surface. There is no trace of the mouth. 

 The furrows of the mantle become more distinctly separated, and its 

 margin more closely set with acute tubercles. The first shell appears 

 in the form of narrow bands with irregular margins ; they are seven 

 in number ; the three or four anterior are equal in breadth ; the hinder 

 diminish rapidly. The anterior filaments disappear. 



The animal sometimes swims and sometimes crawls (figs, d & e). 

 The front portion developes into the head, having a mouth, with 

 curved folds in front. The eyes are situated on the sides on distinct 

 protuberances, and consist of pigment spots and lenses (fig. e). The 

 foot has not yet assumed its full size. The gills are not yet visible. 



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