24 THE OCEANIC HYDROZOA. 



so that the distal appendages are the oldest.^ Where the organs are arranged in distinct 

 groups, as in Apohmia, the same rule holds good, and the young groups are formed on 

 the proximal side of the old ones. 



The like is true for the distinct groups of organs (polypites, tentacles, hydrophyllia, &c.) 

 which are arranged upon the coenosarc of the Calycophoridce, the young buds being 

 continually developed on the proximal side of the old ones ; so that close to the proximal end 

 of the camosarc there is a point which may be regarded as a sort of centre of growth, 

 whence new parts are continually being thrust out towards the distal end. 



The nectocalyces of the Calycophoridce, however, do not appear to follow the same 

 law as the other appendages. Eor if the deflexed axis which supports the nectocalyces 

 of Hippopodius is simply the bent-down proximal end of the coenosarc, as the study of 

 its development (Leuckart, Z. N. K., p. 68) would seem to prove, the young nectocalyces 

 certainly originate upon the distal side of the old ones, or, in other words, they diverge 

 from the same centre of growth as the other appendages of the coenosarc, and, con- 

 sequently, they must be successively pushed out in the opposite direction. 



If, again, Gegenbaur is right in considering the first-formed nectocalyx of DipJii/es to 

 be the distal one, then the locomotive appendages of this genus would follow yet another law, 

 the second being developed on the proximal side of the first formed, while the third bud, 

 destined to replace the first or distal nectocalyx, is certainly produced from the hydrosoma 

 on the distal side of the first, and the successor of the third again arises on its distal side, and 

 so onwards. If, on the other hand, the first-formed nectocalyx of Biphyes is really tlie 

 proximal one of the adult, this genus follows the same law as Hipjmpodius, the young 

 nectocalyces being developed on the distal side of the old ones, while all the other appendages 

 are developed on the proximal side of the old ones, and the two sets thus diverge from a 

 common centre of growth. 



However this may be, the knowledge of the fact that new appendages and parts of 

 appendages are constantly being formed in definite localities is of great importance to 

 the student of the Ilydrozoa ; for he is thereby enabled to study their development as easily 

 on adults as on embryos, and to find in almost every specimen a complete series of stages 

 of every organ. The result is that this part of our knowledge of the Ilydrozoa is tolerably 

 complete. 



Every appendage (except the hydrothecse and lithocysts) commences its existence as a 

 csecal process of the ectoderm and endoderm, the latter being very generally, if not always, 

 ciliated upon its surface, and containing, of course, a diverticulum of the somatic cavity. 



If the appendage be a polypite, it gradually elongates, assumes its characteristic form, 

 and eventually opens at its extremity. The villi, where they exist, are developed as 

 outgrowths of the endoderm. The polypite is at first a perfectly simple sac, and the ultimate 

 division into peduncular, gastric, and buccal portions is the result of the different form 

 assumed, and histological difl"erentiation undergone, by its corresponding regions. 



The pyloric valve of the Calycophorida is formed by the inward growth of the walls 

 of the polypite at the junction between its peduncular and gastric divisions. 



^ Whether the hydrophyllia invariably follow this law may be doubted, and it is not certain that 

 it obtains even for the other organs in such Physophoridai as Velella and Porpiia. 



