FIXED MESSMATES. 61 



Eustoma truncata in its interior. A macrourous crus- 

 tacean which we ought to mention here, the Galathea 

 spinirostris, Dana, frequents a comatula, the colour of 

 which it assumes; it is the same without doubt with 

 the Pisa Stypc, which lives on a polyp known by the 

 name of Melitcea ochracea. 



If we pass from the crustaceans to the molluscs, we 

 have to notice in the first place an elegant gasteropod, 

 the Phyllirhoa bucephala, which carries on its head a 

 singular appendage, the nature of which has only lately 

 been known ; J. Muller took it at first for a medusa, then 

 he abandoned this opinion, when at length Mons. Krohn 

 referred it definitively to the lower polyps; it differs ' 

 from its congeners only by its form, its tentacular cirrhi, 

 and its mode of life : it is the Mnestra parasites. There 

 are a great number of acephalous molluscs, which we 

 might mention as messmates, but we will only refer to the 

 Crenelles which are regularly found in the substance of 

 sponges. 



The Philomechtsa Vogtii of Fr. Muller, which lives on 

 the Haleampa Fultoni, undoubtedly deserves to be men- 

 tioned here as a fixed messmate. Many bryozoa spread 

 themselves over marine animals, and often engage in a 

 deadly struggle with their patron. But among all these 

 bryozoa we must mention an animal very common on 

 the sea-shore at Ostend, and which one would take for a 

 dried leaf, the Flustra membranacea. On the surface of 

 these imitative leaves are found little bouquets of other 

 bryozoa, which are either Grisise or Scrupocellarim. An- 

 other kind, which has also passed for a gelatinous plant, 

 bears the name of Halodactylus. Without any micro - 

 ecopic study, one can obtain an idea of these colonies. 



