28 



POLYPIFERA. 



the office of ovaries. The ova are, in fact, 

 developed in the substance of the longitudinal 

 membranous folds from which the gemmae 

 sprout. As they grow larger they project 

 internally, and soon become pedunculated ; at 

 last, when mature, they detach themselves from 

 the ovigerous fold and fall into the abdominal 

 cavity, whence an issue is afforded to them 

 through the mouth of the polype. No ovule 

 is ever developed from the parietes of the 

 abdominal cavity intervening between the 

 longitudinal folds; and hence there can be 

 but little doubt that these lamellae represent 

 the ovaria of the animal. 



On seeing the same organ producing some- 

 times buds, or gemma;, and sometimes ova, 

 Milne Edwards was led to inquire into the 

 cause of this difference in the mode of repro- 

 duction, which he conceives to be of a me- 

 chanical nature. In those parts of the polype 

 which are not yet imprisoned in the growing 

 mass of the polypary, reproduction is gene- 

 rally effected by the development of external 

 buds, while towards the base of the polypary, 

 where the constituent zoophytes are inti- 

 mately united together by their outer surface 

 and are surrounded by a sort of sheath, no 

 external buds are formed, but the ovules 

 make their escape into the internal cavity of 

 their parent. Hence the distinguished zoolo- 

 gist, whose memoir we quote, is led to infer 

 that, on the one hand, the mechanical ob- 

 stacles to be encountered, and on the other 

 the excitement occasioned by the contact of 

 the surrounding element, determine this dif- 

 ference of procedure, and that the membrane 

 which performs the functions of an ovary 

 produces indifferently either ova or gemmae, 

 according as it finds less resistance or is more 

 stimulated on the inside or the outside of the 

 abdominal walls. 



From the above details it becomes easy to 

 explain how a single polype, by its repro- 

 ductive powers, can form the complicated 

 mass of the compound polypary of the Alcyo- 

 nide, as well as the means whereby an organic 

 continuity is established between all the indi- 

 viduals of the community ; also how the abdo- 

 minal cavity of the primitive individual be- 

 comes common to all the }'oung ones that 

 sprout from it; in short, how the little beings, 

 thus united together, rather resemble a single 

 multiple animal than an assemblage of distinct 

 individuals. But with the advance of age this 

 intimate union gradually diminishes. The 

 communication between the abdominal cavi- 

 ties of the different polypes whose basal por- 

 tions reach as far as the foot of the polypary 

 is first of all interrupted by the ova, with 

 which the lower part of these cavities becomes 

 filled (Jig. 33, /) ; and subsequently, by the 

 pressure of the surrounding parts, the walls 

 become confused, and all communication be- 

 tween the polype whose abdominal tube is 

 thus obliterated and the polype from which 

 it sprung is intercepted. 



The polypary, instead of resembling a tree, 

 all the flowers of which hold together and 

 communicate by common parts, may now be 



compared to a bouquet made by cutting off 

 the more or less branched twigs of a plant 



Fig. 33. 



Alcyonidium elegans. 



1, transverse section of the body of one 'of the 

 polypes, to shoAV the manner in which the eight 

 longitudinal folds are attached around the ali- 

 mentary canal, forming as many longitudinal canals 

 that extend from the abdominal cavity into the ex- 

 tremities of the tentacles. 



2, transverse section of the basilar portion of the 

 polypary, showing the continuation of the abdominal 

 cavities of the polypes Avith their longitudinal folds, 

 and the germs filling them. 



and collecting them in a bundle. The dif- 

 ferent groups of polypes united in the same 

 polypary become thus independent of the 

 neighbouring groups, and, as may readily be 

 conceived, in time each polype can become 

 individualized.* 



The filiform organs (fig. 41, k), situated be- 

 low the digestive cavity, are evidently not 

 ovaria, as they have been considered to be by 

 many authors, seeing that the ova are formed 

 elsewhere ; neither does M. Milne Edwards 

 consider that they can be seminiferous organs, 

 but is inclined to regard them as hepatic 

 vessels. 



In the genus Alcyonium the zoophyte is 

 composed of two principal portions. The 

 common central mass is of a coriaceous tex- 

 ture, porous, and somewhat like cork, being 

 formed of a dense substance, which, when cut 

 into pieces, feels gritty under the knife, owing 

 to the quantity of earthy spicula diffused 

 through its mass. Externally it consists of 

 a reddish granular substance, in which the 

 polype cells are excavated, but internally it is 

 of a grey colour, and permeated by numerous 

 tubes that descend towards the base of the 

 zoophyte, and frequently run into each other. 

 These canals are filled with a gelatinous fluid, 

 and lined with a red material prolonged from 

 the external layer. 



The polypes which stud the surface are as 

 fine as hairs ; but still, with the aid of the 

 microscope, it is not difficult to distinguish 

 the mouth, the vesicular stomach, the muscu- 

 lar envelope of the animal, the ovary, and the 

 glandular organs which depend from the base 

 of the stomach into the abdominal cavity of 

 the polype. Its whole structure has been 



* Raspail, Polypes d'eaux douces. 



