Ill 



MOLLUSCA PARASITIC ON CORALS 



75 



Such a division, however, cannot be rigidly carried out, for 

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

 others are ecto-parasitic in the young form, and become endo- 

 parasitic in the adult. It will be convenient, therefore, simply 

 to group the different forms according to the home on which 

 they find a lodgment. 



On Sponges. — Vulsella and Crenatula almost invariably 

 occur in large masses of irregu- 

 lar shape, boring into sponges. 

 They are especially abundant on 

 Porifera from the Red Sea. 

 Corals form a favourite home of 

 many species, amongst which are 

 several forms of Coralliophila., Rlii- 

 zochilus, Leptoconchus^ and Sistrum. 

 RJiizocMlus is a very singular crea- 

 ture, inhabiting branching corals. 

 When adult, it forms irregular 

 shell}^ extensions of both the inner 

 and outer lips, which adhere to the 

 shafts of the coral, or to the sur- 

 face of neighbouring shells ; at 

 length the aperture becomes com- 

 pletely 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 Red Sea and Indian 

 Ocean, in the young form is shaped like a small Bucclnum. 

 As the coral {Meandrina) to Avhicli it attaches itself grows, 

 the Magilus develops at the mouth a long calcareous tube, 

 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 mat- 

 ter. 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 exten- 

 sion of the two extremities of the shell. Pedicularia^ a form 

 akin to Cypraea, bnt with a more patulous mouth, inhabits 

 the common Cor allium rubriim of the Mediterranean, and 



Fig. 29. — Magilus autiquus L. : 

 A, the adult, imbedded in coral, 

 which has been broken away to 

 show the tube ; B, the young 

 (free) form. 



