86 NATURAL SCIENCE. August. 



The Worm, the Gastropod, the Coral and the Bivalve. 



Strange as any fairy story is this other true tale of the sea that 

 Mr, E. L. Bouvier tells in Comptes Reiidns for July, 1894 (vol. cxix., pp. 

 96-98). Once upon a time it was supposed that certain Madrepore 

 corals, named Heterocyathus a.nd Heter op sammi a, huilt their skeleton around 

 the shell of a living shell-fish, which prolonged its naturally-coiled 

 shell in the form of a tube through the enveloping coral ; and this was 

 the opinion of Deshayes, Milne-Edwards, and Haime. This, however, 

 was not quite correct, as Mr. Bouvier is now able to prove from 

 material collected at Aden by Dr. Jousseaume. The curious facts are 

 as follow. 



The polyps of Heterocyathus and Heteropsammia fix themselves, 

 probably at the close of embryonic life, on the minute but empty 

 shells of various species of Gastropods. As soon as each shell is 

 chosen by a polyp, it receives a new guest in the person of a young 

 worm called Aspidosiphon, one of the Gephyreans. The little worm 

 enters the shell and rolls itself in a corresponding spiral. Then the 

 coral outside and the worm inside grow simultaneously. On the one 

 hand, the coral soon completely covers the shell and threatens the 

 imprisonment of the worm ; but the worm, on its part, continues to 

 grow in the form of a loose spiral, and thus prolongs the coiled cavity 

 of the shell through the tissues of the coral, passing to the exterior 

 by a round opening. At the same time, the worm secretes a tube, 

 which appears at iirst to be an actual continuation of the shell, but 

 which differs from it by its less thickness, its more intimate union 

 with the skeleton of the coral, and by the rougher appearance of its 

 inner surface. In Heteropsammia this tube usually reaches the ex- 

 ternal opening ; but in Heterocyathus it is formed more slowly and does 

 not reach the exterior. This single external opening is, however, 

 not enough to bring aerated water for the respiration of Aspidosiphon, 

 so it sets to work, apparently by a secretion from glands in its skin, 

 to dissolve long perforations through the substance of the coral. 



The worm has a long proboscis, terminated by a crown of short 

 tentacles, and covered with several rows of pointed hooks. It can 

 thrust out this proboscis to catch its prey, and by sticking its hooks 

 in the ground, can, as Dr. Jousseaume has seen, drag along itself 

 and partner. The worm has also two stout horny shields, one of 

 which is placed near the anus at the base of the proboscis, and, when 

 the latter organ is drawn in, it serves to close the opening of the 

 tube. The other shield is at the other end of the animal, and its 

 particular use is not known. The Aspidosiphon that lives with 

 Heteropsammia is a different species to that living with Heterocyathus. 



But we are not at the end of the story yet. A home having 

 been thus prepared by Aspidosiphon, a little bivalve mollusc called 

 Kellia, which seems given to these lazy habits, comes and takes 

 lodgings in the depressions of the tube, and boards on the food that 



