MOLLUSCA PARASITIC ON CORALS 



75 



Such a division, however, cannot he rigidly carried out, for certain 

 forms are indifferently endo- and ecto-parasitical, while others 

 are ecto-parasitic in the young form, and hecome endo-parasitic 

 in the adult. It will he convenient, therefore, simply to group 

 the different forms according to the home on which they find a 

 lodgment. 



On Sjwnges. — Vulsella and Crenatula almost invariably 

 occur in large masses of irregu- 

 lar shape, horing into sponges. 

 They are especially abundant on 

 Porifera from the Eed Sea. 

 Corals form a favourite home 

 of many species, amongst which 

 are several forms of Corallio- 

 2^hila, FJtizocliilus, ZejitoconcJius, 

 and Sistrnm. EMzochilus is a 

 very sino-ular creature, inhahitinu; 

 branching corals. AVhen adult, it 

 forms irregular shelly extensions 

 of both the inner and outer lips, 

 which adhere to the shafts of the 

 coral, or to the surface of neigh- 

 boiu"ing shells ; at length the aper- 

 ture becomes completely closed with the exception of the 

 siphonal tube, which becomes long, and consists of the same 

 shelly material. The common Magilus (Fig. 29), from the 

 Eed Sea and Indian Ocean, in the young form is shaped like 

 a small Bucciiium. As the coral (Meandritia) to which it 

 attaches itself grows, the Magilus develops at the mouth a 

 long calcareous tul)e, the aperture of which keeps pace with 

 the growth of the coral, and prevents the mollusc from being 

 entombed. The animal lives at the free, or outer, end of the 

 tube, and is thus continually shifting its position, while the 

 space it abandons becomes completely closed by a mass of solid 

 calcareous matter. Certain species of Ovula inhabit Gorgonia, 

 assuming the colour, yellow or red, of their host, and, in 

 certain cases, developing, probably for prehensile purposes, a 

 pointed extension of the two extremities of the shell. Fcdic- 

 ularia, a form akin to Cyi)raea, but with a more patulous 

 mouth, inhabits the common Coralliani ruhru/ii of the Mediter- 



FiG. 29. — Magilus antlqnus L. : 

 A, the adult, imbedded in coral, 

 which has been broken away to 

 show the tube ; B, the young 

 (free) form. 



