ACALEPH^. 



183 



Larval Cyansea. 



tability, the larva sometimes elongating, sometimes contracting itself. 

 Four other tubercles bud out in the interspaces of the first four, and 

 all increase in length. These eight arms have the power of remark- 

 ably shortening and lengthening themselves, as exemplified by the two 

 outlines of the same polypoid larva, at a a and b b, Jig. 83. Their 



superficial cilia create vor- 

 tices in the surrounding water, 

 which carry the nutritive 

 molecules to the mouth of the 

 larva, now metamorphosed 

 into an eight-armed naked and 

 solitary polype. By the sub- 

 sequent development of new 

 arms from the interspaces of 

 the old ones, a many-armed 

 polype results, and the type of 

 the hydra is exchanged for that 

 of the actinia. The tentacula 

 are very like those of the ovaria 

 and testis in the adult Medusce; they are ciliated, but not in two regular 

 rows, as in Flustra and Alcyonella. They contain clear corpuscles, 

 or thread-cells, arranged in regular bracelets, like the tentacles on the 

 margin of the disc of fully developed medusae. The mouth of the po- 

 lypoid larvce is very contractile and expansible : they feed on infusoria 

 and on their infusory-like younger brethren, one-half of the body of 

 one of which may often be seen hanging out of the devourer's mouth. 

 If nourishment be abundant the larval polype propagates by gemma- 

 tion {fig. 88.). 



Still more remarkable to Siebold was the production in a few of 

 these larvae of lateral branches from the body, of a great lengthy — 

 in one case three such stolones were developed. These branches have 

 continuations of the digestive cavity in them, and contract and elongate 

 like ordinary arms. If these were irregularities, and the ordinary 

 metamorphoses were delayed, it was, he conceived, through the in- 

 fluence of captivity. Siebold also thought that the larval medusas, which 

 in autumn, in open sea, thus change to fixed polypes, hardly would con- 

 tinue in that state through the winter, but about the beginning of the 

 stormy period would change to free-swimming medusa, and settle 

 down to tranquil depths of ocean. Sir J. G. Dalyell, however, has 

 determined the period of the duration of the polype form of the larval 

 medusae to be much longer than Siebold conjectured, and he even 

 succeeded in keeping a colony of these larva3, which he called " hydrce 

 tubae," for six years. 



N 4 



