HYDROZOA CRASPEDOTA. 775 



Wien, v. 1884. According to Chun, the nectocalyx of the genus Monophyes is a 

 second nectocalyx; cf. op. cit. (i), 1885 ; and Glaus, Z. A. viii. 1885. 



A Siphonophoran has been variously regarded (i) as an assemblage of organs, 

 or (2) as a colony of polymorphic zooids. The former view appears to have been 

 based upon the idea that in any Hydrozoan colony the several factors represent 

 organs of an individual, the individual being the colony itself. Such a view cannot 

 be regarded as any longer tenable. There can be no reasonable doubt that a 

 Hydroid colony is an assemblage of zooids which remain connected instead of 

 separating after their formation by budding. It is no less certain that in a Siphono- 

 phoran, the nectocalyx, pneumatophore, sexual zooid, a polypite or hydrocyst, re- 

 present so many polymorphic individuals, and that structurally a Siphonophoran is 

 as much a colonial organism as a Sertularia *. 



As to the special character of the colony two views have found acceptance. 

 One is, that the coenosarc, as it is called, is the homologue of the manubrium of a 

 Medusa, the primitive hydrophyllium, present, e. g. in the larval Physophora, repre- 

 senting the umbrella : the other that it is a floating colony, the individuals of which 

 are connected by a coenosarcal stem or stolon the equivalent of a hydrorhiza. The 

 former theory is supported by the analogy of such a medusa as Sarsia or Witlia, in 

 which the manubrium produces medusae by budding. The following objections, 

 however, appear fatal to it. (i) A primitive hydrophyllium is not present in all Physo- 

 phoridae, and is not present at all in Calycophoridae, and to suppose that it has been 

 aborted in these instances is the purest assumption; (2) that the hydrophyllium 

 itself is probably the homologue of a medusa in its entirety ; (3) that it is a lateral 

 appendage invariably of the coenosarc, and there is no reason to suppose it is any- 

 thing else in the planula; (4) taking such a form as Epibulia (Calycophore) or 

 Halistemma (Physophore), it is clear that the coenosarc is attached to the aboral 

 apex of the exumbrella of the first nectocalyx developed, and cannot possibly repre- 

 sent a manubrium. For these reasons the second view mentioned above is certainly 

 to be preferred. The coenosarc of a Siphonophoran is then comparable to the 

 aboral stolon of the young Narcomedusan Cunina rhododactyla, or of Cunocantha 

 octonaria, which produces buds for a stated period 2 . The fact that the planula 



1 The hydrophyllium is usually regarded as the homologue of a nectocalyx. Its central endo- 

 derm canal is furnished with accessory canals such as occur in the nectocalyces of Diphyes and Praya. 

 The homology of the tentacle is more difficult. In the Discoideae it probably represents a zooid, 

 i. e. is a dactylozooid, but in Physalia it is possible that the sac represents a hydrocyst. The tentacle 

 itself is in that case attached to this hydrocyst in the same position, i. e. at its base, as is the tentacle 

 of most other Siphonophora. The position is a most unusual one, but Metschnikoff states that he 

 has observed a single tentacle attached similarly to the base of the manubrium of a Medusa 

 {Dipurena). The Siphonophoran tentacle, however, is sometimes attached directly to the coenosarc 

 as in Athorybia rosacea. And it may be noted that in the Physophirid Stephanospira insignis the 

 hydrocyst bearing the female zooids is prolonged apically into a tentacular process with lateral 

 branches bearing nettle-batteries ; see Gegenbaur, Nova Acta, xxvii. p. 399. It is quite possible 

 that such a structure as the tentacle of Porpita, beset with capitate processes, represents a zooid form 

 from which the more complicated tentacles of other Siphonophora have been derived. In determining 

 such a question, much stress cannot be laid on the origin of the tentacles from separate buds. The 

 tentacles of a Hydra originate in the same manner. The place of origin of the buds is however 

 another matter. 



a See Metschnikoff, Z. W. Z. xxiv. 1874, pp. 28 et seqq., PI. V. Figs. 4-8; and Brooks, Mem. 

 Boston Soc. Nat. Hist. iii. (la), 1886, pp. 362 et seqq. 



