XI 



AN ACCOUNT OF RESEARCHES INTO THE ANATOMY 

 OF THE HYDROSTATIC ACALEPH/E. 



British Association Report 1851,//. 2, pp. 78-80 



The observations upon which this communication is based were 

 made during the circumnavigatory voyage of H.M.S. Rattlesnake, but 

 for the most part in the seas which border the coasts of North-eastern 

 Australia, New Guinea, and the Louisiade archipelago. 



With the exception of the mere external form, but very little has 

 been known hitherto with regard to either the Diphydje or the Physo- 

 phoridae,the two families ofwhich the 'Hydrostatic Acalephae' of Cuvier 

 consist, although they are some of the most abundant of pelagic 

 creatures. Indeed, hardly any one can have made a voyage to the 

 East Indies or Australia without being struck with the immense 

 shoals of the Physalia and Velel/a, through which the ship sometimes 

 sails for days together. 



The chief mass of one of the Diphydse is formed by two trans- 

 parent crystalline pieces, which look, when taken out of the water, 

 like morsels of cut glass. One or both of these pieces contains a wide 

 cavity, lined by a muscular membrane, by the contraction of which 

 the animal is propelled through the water. The attachment of the 

 posterior piece to the anterior is very slight, and when detached it will 

 swim about independently for hours together. It was this circum- 

 stance which led Cuvier to consider the two pieces as two distinct 

 animals. 



In the Monogastric Diphydae a single polype is developed in a 

 special cavity of the anterior piece. In the Polygastric Diphydae, a 

 long chain of such polypes, each enveloped in a little transparent 

 "bract,"' occupies a similar position. These polypes have no oral 

 entacles, but a long thread-like tentacle, bearing lateral branches, 



