92 INVERTEBRATE MORPHOLOGY. 



brandies, 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 Caravella, 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 Velella 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 

 Velella, 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 Hydromedusce. It has been mentioned 

 as one of the characteristic features of the Hvdrornedusne that 



fj 



the reproductive elements arise in the ectoderm. They reach 



