HOR NELL— THE INDIAN CONCH 7 



a manifest impossibility. The post-larval rate of growth is quite unknown. On grounds 

 where the food supply is abiindant, it appears to be very rapid, for beds which are ex- 

 hausted of adult shells measuring over 21 inches diameter in one year are found the 

 succeeding year with plenty of shells, 3 inches and over in diameter. 



The chank is an excellent instance of the acquisition by an animal of characters which 

 appear for all practical purposes absolutely perfect to enable it to hold its own with 

 ease in its struggle for existence. Against every one of its known enemies it has evolved 

 suitable means of defence. The massive strength of the shell protects it from the attacks 

 of all ordinary fishes ; the density and thickness of its periostracum give during youth 

 and maturity adequate protection against the insidious attack of the boring sponge, 

 Cliona, and its shell-burrowing congeners ; the strong capsule it constructs for its young 

 gives them adequate protection till they reach a self-supporting stage endowed even at 

 this early period with a fairly strong and resistant shell — one cannot crack it between 

 one's finger and thumb. Its semi-burrowang habits give it great protection against 

 those fishes which have the habit of snapping oif the protruded feet of gastropods. Finally 

 the pale yellowish-brown periostracum assimilates closely in colouring to the sand and 

 should be a further protection against its discovery by foes ; to this form of protection 

 I am, however, not inclined to assign great value, for chank divers can distinguish 

 the presence of a chank even when half buried in the sand, and if they can, I feel 

 assured that predatory fish are equally clever. 



On rare occasions chank shells are found with the larger whorls crushed in ; this is 

 believed to be the work of the great goggle-eyed Ray, Rhinoptera adspersa, which has 

 the most powerful milling teeth of any Indian ray or shark. 



As chanks grow old, their resisting powers diminish, the protecting periostracum 

 receives damage and the burrowing sponge Cliona obtains a lodgment in the shell. Once 

 there, it runs its branching tunnels everywhere in the substance of the shell, converting 

 it into a honey-comb mass. I greatly doubt if this contributes except very occasionally 

 to the death of the chank. This probably occurs usually from senile decay on beds 

 that are not fished commercially. It is noteworthy that beds which have not been 

 fished for some years, contain great numbers of Cliona-burrowed shells, whereas on beds 

 fished regularly the proportion of " wormed " shells is so low as to be practically non- 

 existent. 



