37 



is probably in some exceptional condition. In addition 

 to these features tbe cells have long cilia-like processes 

 which are almost as long as the cells themselves, and 

 these meet in the centre of the sj^ace. In the Pearl 

 Oyster (Margaritifera vulgaris) (2). where there is a well- 

 developed byssus, the same threads are present in 

 connection with the secreting cells of the byssus, and they 

 pass into the horny mass which appears to have been 

 secreted round them. No byssus is present in P. maximus, 

 but these cells are presumably the gland cells, and the 

 peculiar cilia-like processes are identical with those 

 forming the root of the byssus in the Pearl Oyster. 



With regard to the functions of the foot, which seems 

 so rudimentary in the adult, we have already seen that 

 in the early life history it functions as a locomotive organ, 

 while later on it becomes an organ for attaching the 

 byssus threads to a foreign body. In the adult it seems 

 from observations to perform neither of these functions, 

 though in the case of P. opercidaris, the time during 

 which byssal threads are formed is probably long, and in 

 P. varius the byssus is spun and fixed even in the adult 

 stages. 



That the foot can be moved about and extended is 

 easily seen when watching the living animal, and in one 

 case it was observed that this extension brought the apex 

 of the foot outside the valves altogether. It probably is 

 of use, therefore, for freeing the palps and gills of foreign 

 particles, as the foot of the pearl oyster has been observed 

 to do. The cavity at the apex always contains a quantity 

 of mucus, and we have already seen how well the foot is 

 provided with glands. This mucus may be secreted for 

 the purpose of entangling the food particles that are 

 wafted to the mouth, by the gills and palps. 



