92 INVERTEBRATE MORPHOLOGY. 



branches, each richly provided with nematocysts. Associated 

 with this nutritive individual is usually a reproductive form, 

 which in some cases may take the form of an Anthomedusa, 

 separating from the colony and leading a free life, as in 



Velella, or may be medusoid, presenting a medusa form, but 

 lacking a mouth and tentacles and never separating from the 

 colony, or finally a gonopolyp (r) may occur which bears 

 numerous much-degenerated medusoid buds. In some forms 

 there is still another form of individual (s), resembling a tro- 

 phopolyp, but being destitute of a mouth and having a sim- 

 ple tentacle without the secondary branches. From its great 

 sensibility to stimuli this is supposed to be a sensory polyp. 



In some forms, such as Diphyes, no pneumatophore occurs, 

 but nectocalyces are present ; in others, as Agalma, both occur 

 and the colonies resemble somewhat the diagrammatic form 

 described ; while in a third group, including the Portuguese 

 man-of-war Oaravella, the pneumatophore becomes largely 

 developed and nectocalyces are wanting, the stolon at the 

 same time being contracted to a disk lying on the lower sur- 

 face of the pneumatophore. In VeleUa and Porpita the stolon 

 is reduced to a disk, but the pneumatophore is wanting. 



Alternation of generations of a typical form, complicated, 

 however, by the polymorphism, occurs in such forms as 



VeleUa, which possess a free-swimming medusa ; in the ma- 

 jority, however, it is obscured, as in many Tubularian hy- 

 droids, by the greater or less degeneration of the medusa 

 An alternation of another kind, however, occurs in some 

 forms, the bunches of individuals separating from the stolon 

 and leading for a time an independent existence, during which 

 their medusoid reproductive individuals become mature. 



The complicated polymorphism of the Siphonophore colonies leads to a 

 merging of the individualities of the component individuals in that of the 

 entire colony, a process which reaches its highest pitch in such forms as 

 Velella. The various polyp and medusa forms of the colony may be con- 

 sidered as organ-individuals, and by their integration an individuality of a 

 higher grade — a metamere-individual — is produced. 



Development of the ITydromedicsce. — It has been mentioned 

 as one of the characteristic features of the Hydromedusse that 

 the reproductive elements arise in the ectoderm. Th^y reach 



