L06 



NATURAL HISTORY. 



describes a species 

 considerable changes 



occurring 



there in which parasitism has produced 

 Its proboscis has become greatly elongated and 



adapted for piercing the skin 

 of the sea-cucumber, on which 

 it lives, while through disuse, 

 its lingual ribbon, eyes, etc., 

 have entirely disappeared. This 

 degeneration is carried to its 

 farthest extent in Entoconcha, 

 a genus which occurs in Sy- 

 napta, in the Mediterranean. 

 The adult is strikingly worm- 

 shaped, and lives attached, as 

 shown in our figure, to the ali- 

 mentary tract of its host. In- 

 deed, were Ave to study the 



7 x/ 



adult alone, there would not 

 be the slightest suspicion of its 

 being a mollusc ; but its young 

 are undoubted molluscs. Thev 

 have every appearance of young 

 molluscs, — the ciliated swim- 

 ming-lobes, the spiral shell, and 

 all. The life history of Ento- 

 concha is unknown. The adult 

 and young are familiar; but 

 how the young obtains entrance 

 to the body cavity of the host 

 has not yet been discovered. 



On a preceding page worms 



were mentioned which built a 



calcareous tube much like a 



mollusc shell, while among the 



molluscs there are forms whose 



shells are even more irregular 



than those of some of the 



worms. Vermetus is the name 



of the genus, and it is very 



appropriate, as it implies the vermiform character of the shell of these 



aberrant forms. In early life these molluscs start off as well-behaved 



molluscs should, and build a regular spiral shell; but with increase in age 



Fig. 100.- - l h> Left-hand figure shows a Synapta in outline, 



with the parasitic Entoconcha attached to the alimentary 



canal; the figure on thfi right represents the same, en- 



is'iryos being seen through the walls of the 



coiled i>;ir;isite. 



