PARASITES ON CORALS. 



335 



itself round in a spiral. At the same time it appears so to 

 stimulate the base of the coral that it grows faster than the cup 

 itself, and thus the base gradually but conspicuously outgrows 

 the cup. Many corals are affected in a perfectly similar manner 

 by parasitic crustaceans ; Diaseris Freycineti by certain Cirrhi- 

 pedia of which the shells often greatly Outgrow the foot of the 

 Diaseris, thoiigh this too is abnormally extended. Certain 

 species of the genus Heterocyathus also are infested by Sipun- 

 culidiE just like Heteropsammia, and their growth is modified 

 by them. Even in fossil species of this genus holes are often 

 observable in the foot which can scarcely have been anything 

 else than the dwellings of Sipunculidse."'' There are also, it is 

 true, some species of the same genus, Heterocyathus, which 



Fig. ^Q.—Beteropmmmia Mkkelim. a, seen from above with the broad base ; 6, from 

 below, with the tube of the Aspidosiphon partly opened ; c, also from below but intact, 

 showing the large entrance of the tube and the small side openings. 



establish themselves on the shells of univalves ; the animals 

 which have formed these are invariably dead, and the cavity of 

 the dead shell is always occupied by another of the Sipuncu- 

 lidce ; but in this last case the worm has bad no effect on the 

 growth of the coral, which has followed the normal course of its 

 species and is at most so far modified that the coral, in order to 

 obtain as secure a hold as possible, has extended its base rather 

 more widely over the surface of the shell than seems to be its 

 natural habit. 



In the genera Heteropsammia and Heterocyathus, in the 

 second place, another generic character becomes modified in a 

 very peculiar manner by the Sipunculidce. All the species 

 attacked, of both genera, display both on the under side of the 



