CCELENTERATA. 



135 



layers have a share), which become converted into the hydrophyllia, 

 nectocalyces etc. The main oral part of the planula becomes generally 

 converted into the polypite, though in some instances (Crystalloides) 

 it remains as a yolk-sack, and 

 only secondarily gives rise to a 

 polypite. 



Two very different views 

 have been taken as to the na- 

 ture of the various component 

 parts of the Siphonophora, and 

 the embryological evidence has 

 been appealed to by both sides 

 in confirmation of their views. 

 By Huxley and Metschnikoff the 

 various parts nectocalyces, hy- 

 drophyllia, hydrocysts, polypites, 

 generative gonophores etc. are 

 regarded as simple organs, while 

 by Leuckart, Haeckel, Glaus etc. 

 they are regarded as so many 

 different individuals forming a 

 compound stock. The difference 



between these two views is not FlG 77 LAKVA Qy CRYSTALLOIDES . 

 merely as to the definition ot an (After Haeckel.) 



individual 1 . The question really li.ph. hydrophyllium ; h. hydrocyst; t. 



is, are these parts originally de- tentacle; pp. pneumatophore ; 2)0. polypite; 

 rived by the modification of com- y ' yo 



plete zooids like the gonophores and trophosomes of the fixed Hydrozoa 

 stocks, or are they structures derived from the modification of the 

 tentacles or some other parts of a single zooid ? 



The difficulty of deciding this point on embryological evidence 

 depends on the fact that ontologically a tentacle and a true bud arise 

 in the same way, viz. as papilliform outgrowths containing prolonga- 

 tions of both the primitive germinal layers. The balance of evidence 

 is nevertheless in my opinion in favour of regarding the Siphonophora 

 as compound stocks, and the views of Glaus on this subject (Zoologie, 

 p. 271) appear to me the most satisfactory. 



The most primitive condition is probably that like Physophora in an 

 early stage with an hydrophyllium enclosing a polypite (cf. Haeckel and 

 Metschnikoff). In this condition the whole larva may be compared to 

 a single Medusa in which the primitive hydrophyllium represents the 

 umbrella of the Medusa, and the polypite the manubrium. The tentacle 

 which appears so early is probably not to be regarded as a modified zooid, 

 but as a true tentacle. The absence of a ring of tentacles is correlated 

 with the bilateral symmetry of the Siphonophora. 



1 From the expressions used by Huxley, Anatomy of Invertebratcd Animals, p. 149, 

 it appears to me possible that his opposition to Leuckart's view is mainly as to the 

 nature of the individual. 



