rOLYZOA INFUNDIBULATA. 257 



nature of their respiratory organs. Interior and immotive in 

 the one tribe, they line, in a reticular pattern, the parietes of 

 a sac capacious enough to contain a sufficiency of the aerating 

 fluid ; while in the other they clothe the exsertile tentacula 

 in the form of cilia which must he placed outwards amid 

 the circumfluent waters before they can play and fulfil their 

 functions. 



The Polyzoa never occur in a separate and naked form, 

 but are always placed within the cells of a polypidom of 

 a calcareous, membranous, or fibro- gelatinous consistence ; 

 and of so many dissimilar figures as to render a general descrip- 

 tion of it impossible. The form of the cells in many genera, 

 as in Eschara, Flustra, and Cellepora, suggests a belief that 

 their tenants, although arranged in a close and determinate 

 manner, are each separate from their neighbours and com- 

 plete in themselves, an opinion that was held by some of 

 our best naturalists ; but the observations of Dujardin on 

 some allied fossil polypidoms, render it very probable that 

 there are pores of communication between the cells ; * while 

 those made by Professor Grant seem to have proved that the 

 polypes of the Flustra are connected together by a living 

 axis, and are hence truly compound beings. Since the Vesi- 

 culiferse also, which are admitted to be composites, belong 

 unquestionably to this remarkable form of animated entities, 

 it is safer, for the present, to consider all the Polyzoa as 

 compound polypes.^ There is, nevertheless, a remarkable 

 difference between them and the Anthozoa in their mode 

 of composition. In the latter the polypes are simply de- 

 velopments of the common central fleshy mass, identical 

 with it in structure and texture ; in the former each indi- 



* Blainville's Actinologic, p. 675. See Ann. des Sc. Nat. vi. p. 320. 



t " The polypi are most intimately and inseparably connected with the axis by 

 three parts of their body, and are only digestive sacs or mouths developed by the 

 axis, as in all other zoophytes, for the nourishment of the general mass. By the axis 

 of a zoophyte, I understand every part of the body excepting the polypi, whether of 

 a calcareous, horny, or fleshy nature. The exact mathematical arrangement and 

 forms of the cells of Flustrae is incompatible with their existence as separate and 

 independent beings, but is quite analogous to what we are accustomed to observe 

 in Cellariae, Sertulariae, Plumulariae, and many other well-known compound ani- 

 mals." Grant in Edin. New. Phil. Journ. iii. 116. See also Blainville, Man. 

 d'Actinologie, p. 99. 



