66 ANTHOZOA. 



upon one stock. There is, however, a very important difference ob- 

 servable between these two genera of zoophytes, in other respects so 

 similar. In the Alcyons the abdominal cavity of the young polyps is 

 not directly continuous with the abdominal cavity of their parent, and 

 it is only by the intermedium of the vascular system, described above, 

 that they are placed in communication with each other a modification 

 which depends upon a difference in the mode of formation of the repro- 

 ductive gemmae. When an Alcyon stock is about to put forth a new 

 branch, the spongy part of the polypary (that portion which is formed 

 by the external tunic of the polyps, and permeated by the vascular 

 network) begins to increase in size at some determinate point of its 

 periphery, and soon produces a tubercle of greater or smaller size, into 

 which the vessels spoken of above are continued, and form numerous 

 anastomoses with each other. At this early period of development the 

 new branch presents no trace of polyps ; but its vascular tissue is never- 

 theless already studded with calcareous crystals, and exactly resembles 

 that situated in other parts of the common mass, between the abdominal 

 cavities of the adult polyps ; it must therefore necessarily be traversed 

 by the currents which circulate in the general vascular system. On 

 dissecting one of these newly formed branches, the vestiges of young 

 polyps may be distinguished; and if the sprouts examined are still 

 further advanced, it is easy to distinguish the young animals within, 

 already possessing the form they will afterwards exhibit, but having 

 not yet established a communication with the exterior (fig. 28, 1). At 

 length, however, this communication is effected ; the newly formed polyp 

 only differs from the pre-existing ones in its small size ; and as it grows, 

 its increase causes the enlargement of the polypary. In this case it is 

 very evident that the part which gives birth to the reproductive gemmae 

 is no portion of the individual polyps of the Alcyon, but is common 

 to them all. The generative tissue surrounds these little beings with 

 a sort of living sheath, and produces in the interior of its own substance 

 new polyps, quite independently of those previously in existence. These 

 polyparies might therefore be compared to a sort of common ovary, the 

 products of which are never completely individualized, but remain per- 

 manently lodged in its substance, and minister to the support of its 

 existence and the aggrandizement of its tissue. 



(143.) This singular mode of reproduction, M. Milne-Edwards ob- 

 serves, seems at first sight to be very different from that observed in 

 the Alcyonidium; but on reflection, a considerable analogy may be 

 traced between them. In Alcyonidium the internal tunic of the ab- 

 dominal cavity fulfils the functions of an ovary, and produces at deter- 

 minate points both gemmae and ova; in Alcyon, on the contrary, 

 while the internal membranous layer gives birth to ova, the gemmae are 

 developed elsewhere, from the canals which permeate the common mass, 

 the membrane which forms these canals, and which is the seat of 



