ACEPHALA. 



than four or five can be completed in a day and 

 night. Every thread is tested as soon as made, by 

 the swinging of the shell as far as those already fas- 

 tened will allow, by which perhaps the thread may 

 be slightly stretched. According to Reaumur, the 

 animal exercises this instinct, even from its earliest 

 infancy, for he saw some of them forming threads, 

 when the whole shell was not larger than a millet 

 seed. 



Another genus which forms a cable of byssus, and 

 which is interesting in other respects, is 



Tridacne,* the Clamp, 



marked by the three openings with which the 

 mantle is perforated, one for the passing out of the 

 foot, the second for respiration, and the third for the 

 evacuations. The animal is remarkable for not being 

 placed in the shell, but, as it were, partly thrust 

 out in front. Some species attain a vast size, as the 

 enormous Clamp of the Indian seas, (T. Gigas,) 

 whose shells alone sometimes weigh three hundred 

 pounds, and measure four feet in width. It is 

 marked with high and broad ribs, relieved by pro- 

 jecting scales. Its vast weight is sustained by a 

 strong cable of byssus, which passes through an 

 opening below the hinge, and by which it is sus- 

 pended to the rocks, however heavy, and securely 

 fixed out of the reach of storms. So tough and 



* T>s/j, treis, three, and IO.KVU, dakno, to bite, as if bitten in three 

 places. 



