758 THE ANIMAL KINGDOM. 



tentacles as in Podocoryne, the latter reduced to ectodermic knobs in Hy- 

 dractinia, or tentacles and no mouth as in Myriothela ; or it is, as is more 

 commonly the case, e. g. in all Campanulariae, destitute of both structures. 

 (2) The Dactylozooid, a mouthless hydranth, modified for solely defensive 

 and offensive purposes. Such zooids are universal among Hydrocorallina. 

 They are long and filiform in the Milleporidae, and bear a variable number 

 of short scattered capitate tentacles, but in the Stylasteridae are devoid of 

 tentacles. A circle of them surrounds each gastrozooid of Millepora and 

 some Stylasteridae. They communicate by several apertures with the 

 coenosarc. Under this heading should also be included the three following : 

 (i) The spiral zooids of Podocoryne, mouthless and tentaculate, tubular in P. 

 carnea, with a solid endodermal axis in P. Haeckelii : the similar zooids of 

 Hydractinia, which are tubular and possess small ectodermic tentacles *. 

 (ii) The highly extensile tentacle-like and apparently solid zooids of the 

 Campanularian Ophiodes y attached singly to the hydrocope or in numbers 

 to the hydrorhiza. They are capitate, and the terminal knob supports 

 numerous cnidoblasts. There appear to be very similar structures seated 

 on the hydrorhiza of Oplorhiza and Lafoeina and below the hydrothecae of 

 Halecium Gorgonidae 2 . (iii) The structures known as nematophores, sarco- 

 thecae y guard-polypes or macho-polyps which are confined to the Campanu- 

 larian family Plumularidae and are in close relation to the thecae of the 

 ordinary hydranths. They are tentaculoid with a solid endodermal axis, lon- 

 gitudinal ectodermic muscle cells, and are capitate. The knob in one form 

 contains cnidoblasts, sense cells, supporting cells, sub-epithelial ganglion 

 cells, and radial muscles ; see PL xiv, Fig. 8, and p. 330. In some species 

 the cnidoblasts are replaced in the guard polypes situated in front of the 

 hydranths by cells containing adhesive globules similar to the structures 

 so called in Ctenophora. And in the genus Aglaophenia certain guard- 

 polypes of this last-named kind placed behind the hydranths, possess a 

 basal ectodermic thickening with cnidoblasts, ganglion cells, &c., protrusible 

 from an aperture of the theca separate to that by which the macho-polypes 

 themselves are protruded. The macho-polypes are usually disposed in a 

 median and two lateral rows 3 . (3) The Gastrozooid, a term applied to the 



Weismann considers (Entstehung der Sexualzellen, &c. pp. 245-6) that the blastostyle is correlated 

 with the degeneration of the Medusa (pp. 760, 768, post}. A Medusa-bearing blastostyle occurs in 

 Podocoryne alone among Tubulariae, but the Campanularian Medusa is invariably developed from one. 



1 The spiral zooid is confined to the edge of the colony bordering the aperture of the Gastropod 

 shell on which the colony is planted, or of any accidental hole in it. The zooids are found in the 

 colonies of both sexes, not in the male only, as Grobben thought. See Weismann, Entstehung der 

 Sexualzellen, &c. pp. 63-65. 



2 See note, p. 6, in Allman's Plumularidae, Challenger Reports, vii. 



3 Von Lendenfeld states that the guard animal with cnidoblasts is protruded when the colony is 

 disturbed ; the one with adhesive cells, on the contrary, retracted ; and that in the third variety 

 above described the part or head with adhesive cells is protruded for holding fast the prey, the basal 

 thickening, on the contrary, if the colony is roughly disturbed, the two parts being retracted under 



