HYDROZOA CRASPEDOTA. 757 



another as in Sertularidae. The hydranth itself varies somewhat in the 

 shape of the hypostome, the relative proportions of the hydrocephalis and 

 hydrocope, and the sharpness with which the latter is marked off from the 

 former by a constriction or neck 1 . The tentacles of Hydra, Myriothela, 

 and Polypodium (p. 766), are tubular, those of other Hydroids are, with a few 

 partial exceptions, solid, possessing a notochord-like axis of modified 

 endoderm cells generally disposed in a linear series. They are either 

 filiform or capitate. In the former the cnidoblasts are scattered along 

 them in, for the most part, irregular groups containing micro- and macro- 

 cnidia, intermingled ; in the latter they are restricted to the swollen end, 

 and contain almost exclusively macrocnidia. The tentacles are either 

 arranged in a single adoral whirl, all similar in size, or alternately large or 

 small, their bases sometimes united by a thin ectodermic lamella, e. g. in 

 Campanopsis, Laomedea, &c. ; or in two or more whirls of similar members, 

 e. g. Tubularia, or of dissimilar, e. g. Stauridium, Cladonema, Pennaria, 

 Tiarella. Or they are more or less irregularly scattered like the filiform 

 tentacles of Clavidae, and the capitate tentacles of some Corynidae. In 

 Cladocoryne the scattered tentacles are branched as well as capitate. They 

 are usually numerous, but are reduced to two in Lar Sabellarum and 

 Amphibrachmm Euplectellae, to one in Monobrachium, and are absent 

 altogether in Microhydra and Protohydra. 



The hydranth is sometimes modified for special functions, and the 

 following must be regarded as polymorphic forms of it. (i) The blastostyle t 

 a hydranth comparatively rarely developed in Tubulariae, the function of 

 which is solely to produce sexual zooids 2 . It may possess a mouth and 



1 The base of the hydranth of Eudendrium is surrounded by an ectodermal ring containing 

 cnidoblasts separated by a furrow from a ring of gland cells. In E. ratnosum some of the hydranths 

 are furnished with a single basal horn-like process the cnidophore armed terminally with a battery 

 of cnidoblasts, and capable of executing slow movements. See Weismann, op. cit. p. 94, and Mitth. 

 Zool. Stat. Naples, iii. 1882. A glandular ring occurs in some Campanularidans : von Lendenfeld, 

 Z. A. vi. p. 69. 



3 According to Allman (op. cit. ante, p. 336), the blastostyle of Eudendrium is formed by the 

 atrophy of the hydranth, every stage being present in one and the same colony. Weismann, on the 

 contrary (op. cit. ante, pp. 97, 103), states that it differs from a hydranth in development and 

 structure from the first. The hypostome is absent, but the female has a double circle of small 

 tentacles and a minute mouth, organs absent in the male. Both Cladocoryne and Eudendrium 

 ramosum (as well as other species of the latter genus, judging from Allman's figures) are instances in 

 which gonophore-bearing hydranths and blastostyles may be found side by side. The blastostyle of 

 the female Halecium gives origin to two hydranths at its apex (Allman, op. cit. p. 58, Fig. 29, and 

 Hincks, op. cit. ante, i. p. 221). The blastostyle of the thecate Campanularians is expanded distally 

 into a disc, hollow at least in some instances, in which rhythmical contractions, due to the presence 

 of ectodermic muscles, occur in Eucopella, as in the gonophores of Coryne pusilla. The blastostyle 

 in Eucopella and Laomedea is broken up into a number of tubes which open into the terminal hollow 

 disc. The gonophores in Eucopella are borne upon one of these tubes, in Laomedea in connection 

 with a central spadix. See von Lendenfeld, Z. W. Z. xxxviii. pp. 537-544 ; Allman, op. cit. ante, 

 pp. 47-8, and Fig. 20. The homology of the pedicle bearing medusae in Corymorpha, and Monocaulus 

 or gonophores in Tubularia does not seem to be known. It always springs from the hydrocephalis. 



