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. (PI. 

 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 tine tubular net-work with which the spaces between these ca- 

 nals isoccupied, (Fig. 5.) for Milne- Ed wards 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 oviformcs qui imitcnt six ovaires." — Aiiim. s. Vert. ii. p. 4"5 — 7; 

 417. 



♦ Lam. Anim. s. Vert. ii. 46.3. 2de edit. 



