ZOOPHYTA ASTEROIDA. 
171 
them is easily proved to be erroneous, their true office remains 
conjectural. Milne-Edwards says they have great analogy with 
the biliary vessels of insects ; and they probably secrete some 
fluid subservient to digestion and to the more complete assimila- 
tion of the food. 
As already remarked the protrusile portion of the polype is 
very delicate, the internal viscera being as it were enclosed in 
a serous bladder so transparent as to permit a view of their dis- 
position. This envelope is itself, however, composed of two 
very thin membranes in intimate union : at the base of the 
body the outer of these assumes a considerable thickness, and 
in coalescing with that of the adjacent polypes, constitutes the 
common cortical portion into which each animalcule retreats at 
will by a process of invagination, which we have had occasion al- 
ready to compare to that by which a snail shortens its horns. (PL 
xxvi. Fig. 2.) In the greater number of the Asteroida this common 
portion secretes carbonate of lime, which is deposited in the meshes 
of its tissue either in granules or in crystalline spicula, and im- 
parts more or less of consistency to the whole. The inner tunic 
on the contrary continues unaltered, and prolonged within the 
polypiferous mass, it lines the cell, the abdominal cavity, and 
the longitudinal canals which permeate the mass, as well as the 
very fine tubular net-work with which the spaces between these ca- 
nals is occupied, (Fig. 5.) for Milne-Edwards has shewn that there 
is a free communication between these parts through the medium 
of numerous minute apertures perforated in the sides of the ab- 
dominal cavity. * It is probably in this tenuous inner tunic that 
the buds or gemmae by whose increase and evolution the polype- 
mass is enlarged are generated, the shape and size of the mass 
depending upon the manner, or pre-ordained fashion, in which 
the buds are evolved, for in some, as in Pennatula, determinate 
spots only have the appropriated organization ; while in others, 
as in Alcyonium, the generative faculty appears to be undefined 
and diffused. These buds are produced in the net -work of the 
crust ; while the gemmules or ova by which the species is pro- 
varies dans leur forme, selon les genres, paraissent s’arreter a six grappes 
de gemmules oviformes qui imitent six ovaires.” — Anim. s. Vert. ii. p. 405 — 7; 
417. 
* Lam. Anim. s. Vert. ii. 465. 2de edit. 
