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 its small size, and as 

 it grows its increase causes the enlargement 

 of the polypary of which it forms a part. In 

 this case it is very evident that the part which 

 gives birth to the reproductive gemma? is no 

 portion of the individual polypes 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 polypes, 

 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 indi- 

 vidualised, but remain permanently lodged in 

 its substance, and minister to the support of 

 its existence and the aggrandizement of its 

 tissue. 



This singular mode of reproduction, M. 

 Milne Edwards observes, 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 abdo- 

 minal cavity fulfils the functions of an ovary, 

 and produces at determinate points both 

 gemma? and ova; whilst in Alcyon, on the 



contrary, while the internal membranous layer 

 gives birth to ova, the gemma? are developed 

 elsewhere, from the canals which permeate the 

 common mass. But the membrane which 

 forms these canals, and which is the seat of 

 this kind of vegetative reproduction, is merely 

 a continuation of the internal tunic ; and 

 hence it is easy to understand how it may 

 fulfil analogous functions. 



CORALLID^E. The cortical polypes, as they 

 have been named by authors, mainly differ 

 from the Alcyonidae from the circumstance 

 that the fleshy cortex which constitutes the 

 common polypary, instead of being merely 

 indurated by the deposition of earthy spicula 

 in its interior, secretes for itself a solid central 

 axis, upon the ramifications of which it is 

 spread out, and thus enabled to form itself 

 into arborescent expansions, of dimensions 

 such as would be quite unattainable without 

 this arrangement. The composition of this 

 central axis varies in different genera ; some- 

 times it is dense and stony (JMiopkyta) ; 

 sometimes flexible, and composed of a horny 

 substance (Keratophyta) ; this difference is, 

 however, of no physiological importance, for 

 very frequently the two substances are se- 

 creted in the same individual in different por- 

 tions of its substance. The solid element in 

 the Lithophyta is carbonate of lime ; in the 

 Keratophyta it is concrete albumen. 



" A species of Gorgonia, of a black colour 

 and high polish, like black sealing-wax (Anti- 

 Fig. 35. 



Branch of Gorgonia Umbraculum slightly magnified. 



patties?), examined by Mr. Hatchett*, was 

 found, by immersion in dilute nitric acid du- 

 ring 28 days, gradually to become transparent 

 and of a bright brownish yellow. In this 

 softened state it was steeped two days in 

 water, and was then opened longitudinally ; 

 by this the whole structure became apparent, 

 and consisted of thin coats or tubes of a 

 beautiful transparent membrane, which be- 

 ginning from a central point gradually be- 

 came larger, according to the order in which 

 they receded from the centre. These mem- 

 branes were so delicate that the fibrous tex- 

 ture could scarcely be discerned. 



The acid in which these had been steeped 

 was tinged of a very pale yellow. Ammonia 

 being added changed it to a deep yellow or 

 orange colour; but the transparency of the 

 liquor was not disturbed by this or any other 

 precipitants which had been employed in the 

 former experiments. 



* Phil. Trans. 1800. 



