164 



SIPHONOPHORA. 



between these two views is not merely as to the definition of an 



individual 1 . The question really is, are these parts originally 



derived by the modification 



of complete 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 pro- 

 longations of both the primi- 

 tive germinal layers. The 

 balance of evidence is never- 

 theless in my opinion in 

 favour of regarding the Si- 

 phonophora as compound stocks, and the views of Claus 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. 



The primitive zooid of a Siphonophora stock is thus a Medusa. Like 

 Sarsia and Wilsia this Medusa must be supposed to have been capable of 

 budding. The ordinary nectocalyces by their resemblance to the umbrellas 

 of typical Medusae are clearly such buds of the medusiform type. The same 

 may be said of the pneumatophore, which, as pointed out by Metschnikoff, 

 is identical in its development with a nectocalyx. Both are formed by a 



1 From the expressions used by Huxley, Anatomy of Invertebrated Animals, 

 p. 149, it appears to me possible that his opposition to Leuckart's view is mainly as to 

 the nature of the individual. 



FIG. 77. LARVA OF CRYSTALLOIDES. 

 (After Haeckel.) 



//.///. hydrophyllium ; h. hydrocyst ; t. 

 tentacle ; //. pneumatophore ; po. polypite ; 

 yk. yolk -sack. 



