No. 6(l92l) COMMON MOLLUSCS OF SOUTH INDL-\ 



lOI 



They are easily recognized by the jointed shell protectin;^ the 

 back. It is made up of eight distinct plates fitted to overlap one 

 another like a row of roof tiles. When detached from their foot- 

 hold, chitons roll up into a ball like the short millipede or like the 

 armadillo and the hedgehog. All our Indian chitons are sluggish 

 creatures feeding on the smaller seaweeds clothing the adjacent 

 rock surfaces. One outstanding interest they possess is the fact 

 that some species have developed very minute eyes, complete with 

 lens, retina and pigment, upon the surface of the shell plates. 

 They are often thousands in number in the one individual, always 

 most numerous on the anterior plate. Chitons have no economic 

 importance in India. 



Unlike the gastropods proper, the body and organs of chitons 

 are bilaterally symmetric, that is, the right side of the body is 

 exactly like the left. The head is at the front end, and the gullet, 

 stomach, and intestine form a straight tube through the body, 

 ending in the anus at the hinder end. The breathing organs are a 

 series of plate-like gills, arranged on each side between the mus- 

 cular foot and the edge of the thick leathery mantle in which are 

 sunk the eight plates that form the shell. Like the alimentary 

 canal, the heart is straight and tubular as in the ancestors of the 

 bristle worms from which the mollusca may have been derived. 

 These points are of much importance to remember, for we shall 

 find in the gastropoda striking divergences from these simple and 

 probably ancestral characters. 



CLASS II.— GASTROPODA. 

 Sub-class i.— Streptoneura. Order i.— Diotoc.ardia. 



The most primitive and simplest of these show distinct kinship 



with the chi~ 





tons and serve 

 _ " to bridge the 



•-" gap between 



the latter and 

 the spirally 

 coiled shells 

 that character- 

 ize the great 

 majority of the 

 gastropods. Of 



such simpler forms are the LIMPETS {Patellidae). In these 



FlO. I. A Limpet {^Patella riidis). 



