. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 30 POLYPIFERA. developed around the pre-existent polypes, and thus augment indefinitely the number of individuals united upon one stock. There is, however, a very important difference observ- able between these two' genera of zoophytes, in other respects so similar. In the Alcyons the abdominal cavity of the young polypes 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 mo
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 30 POLYPIFERA. developed around the pre-existent polypes, and thus augment indefinitely the number of individuals united upon one stock. There is, however, a very important difference observ- able between these two' genera of zoophytes, in other respects so similar. In the Alcyons the abdominal cavity of the young polypes 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 de- pends upon another difference in the mode of formation of the reproductive 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 polypes 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 de- velopment the new branch presents no trace of polypes, but its vascular tissue is neverthe- less already studded with calcareous crystals, and exactly resembles that situated in other parts of the common mass between the abdo- minal cavities of the adult polypes. It must, therefore, necessarily be traversed by the cur- rents which circulate in the general vascular system. On dissecting one of these newly formed branches the vestiges of young polypes may be distinguished; and if the sprouts ex- amined are still further advanced, it is easy to distinguish the young animals within, already possessing the form they will afterwards ex- hibit, but which have not yet established a communication with the exterior. At length, however, this communication is established, and the newly formed polype only differs from the pre-existing ones in i
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