94 PHOLADIDiE. 



own accord removing from one place to another, in this 

 respect unlike the Solen and other bivalves which make 

 only a temporary sojourn in sand or mud. The extensor 

 muscle, aided by the prickly surface of the shell, serves 

 to keep the Pholas fixed in its case when it rises to the 

 surface in search of food. The depth of the hole exca- 

 vated by P. Candida is between 5 and 6 inches ; its shell 

 and tubes, the latter being fully extended, measure only 

 4 inches. The ascent must be effected by stretching 

 out the foot ; and by contracting it the Pholas can 

 descend to the bottom and retreat for shelter. I have 

 observed the latter fact ; and I believe the above to be 

 the correct explanation of it. The Pholas gets rid of 

 the excavated material by closing the valves of its shell, 

 and forcibly expelling the detritus by a spasmodic action, 

 through the larger or incurrent tube, together with the 

 water contained in the body. The detritus is not re- 

 moved to any distance ; and some of the finer particles 

 are occasionally washed by the waves into the hole, and 

 line its sides. Mr. Osier and M. Cailliaud account in 

 other and different ways for this phenomenon. Accord- 

 ing to the latter naturalist, the maternal care of the 

 Pholas for its young is very peculiar. He says that, 

 like the Gastrochcena, it makes with its acidulated 

 siphons small oval holes in the surface of the calcareous 

 rock which it inhabits, and inserts in these holes a 

 portion of its brood ! This remarkable instinct has its 

 parallel in the case of Teredo, if we place equal confi- 

 dence in everything that Sellius wrote on that subject. 

 Most of the Pholadidce can entirely withdraw into their 

 shells : Pholas crispata is an exception among the 

 British kinds. If a layer of peat, mud, or shale in- 

 habited by Pholades is too thin to contain them, they 

 will either perish or their growth will become stunted. 



