FOOD, HABITS, ETC. 83 



seized be a soft portion such as a fin or a gill-fila- 

 ment, penetrate the tissues, and the irritation they 

 set up causes an overgrowth of the skin of the fish to 

 envelop the Glochidium, which thus cradled thrives 

 for some two to six weeks on the juices of its host. 

 Meanwhile the byssus and adductor muscle of the 

 Glochidium disappear and the two permanent 

 adductors of the adult and the foot are developed. 

 The mantle secretes a new shell, resembling the 

 adult form, beneath the Glochidium shell, which 

 persists, however, for three or four weeks after the 

 young Mussel is again set free by the perishing of 

 the containing cyst. By the time, though, that it 

 quits this temporary abode and proceeds to complete 

 its growth, it has almost certainly been carried far 

 from the place of its origin to some spot where 

 perhaps there will be more room for it. 



Overcrowding is undoubtedly as bad for animals 

 as it is for plants, and in the vegetable kingdom 

 various well-known adaptations are provided to 

 insure the dispersal of the seeds and their convey- 

 ance to fresh soil. Among the Mollusca the instance 

 just cited is perhaps the most distinct example of a 

 like provision. With the marine Mollusca the 

 shifting waters of the sea are practically the sole 

 vehicle for the transportation of the young to new 

 spots, and the embryos of nearly all, including those 

 that, like the Oyster, are fixed in the adult state, are 

 free swimmers, principally on the surface of the sea. 



