HYDROZOA CRASPEDOTA. 759 



digestive zooid of Hydrocorallina. It is relatively short, and large, with a 

 wide mouth, and in the Stylasteridae is sometimes devoid of tentacles. 

 The base of the gastric cavity gives off many communicating tubes to the 

 coenosarc. (4) A spine occurring in the genus Podocoryne alone J . (5) The 

 claspers of Myriothela, simple filiform structures, with a sucker-like ex- 

 tremity, which bear each one an ovum during its development 



In many Plumularidae a ramulus, hydrocladium or theca-bearing por- 

 tion of a branch, becomes modified to form a phylactocarp for the gonothecae 

 which here occupy the position of hydrothecae. This phylactocarp, to 

 which numerous machopolypes are attached, varies in structure and in the 

 degree to which the ramulus is modified. It may form a cage completely 

 open, nearly closed, or altogether closed, e.g. in Aglaophenia pluma, where 

 it receives the special name of corbida^. 



The coenosarc connecting the hydranths and other members of the 

 colony is nearly invariably a simple tube. But in Tubularia indivisa, 

 Corymorpha and Monocaidus, the endoderm cells of the stem proliferate, 

 leaving only tubular passages : and in some Plumularidae the main stem 

 and principal branches are hollow, with a number of peripherally placed 

 endodermal tubes, communicating laterally with one another, all inclosed 

 in a common thin ectoderm and perisarc 3 . See also note 2, p. 755. 



The sexual zooid or Medusa has the typical structure detailed on pp. 

 247-8, and Fig. 1 1 4 . The Medusae of the Hydroidea constitute Haeckel's 

 first sub-legion of Craspedota, the Leptolinae, in which there are either 

 ocelli or eetodermic otocysts. There are two groups, the Anthomedtisae 

 and Leptomedusae. The former possess ocelli and never otocysts : their 

 generative products ripen in the walls of the manubrium, and their radial 

 canals usually number four, very rarely six to eight, and they are derived 

 from Tubularian Hydroids. The latter may have ocelli, but generally 



reversed conditions. He also observed that the third variety has at first cnidoblasts in its terminal 

 knob, but that during growth the nematocysts are lost, their cells undergo degeneration, and certain 

 supporting cells develope adhesive globules. Metschnikoff has observed the ectoderm of the macho- 

 polyps in two species of Plumularia ingest not only carmine suspended in the water, but the dead 

 hydranths of the colony : see Q. J. M. xxiv. p. 94. 



1 Spines also occur in Hydractinia ; but, judging from Allman's figure, their structure is unlike 

 that of the spines of Podocoryne. 



a It is impossible to explain the structure of the phylactocarp without figures. For details, see 

 Allman, Plumularidae, Challenger Reports, vii. pp. 10-15 '> an d f r tne development of the corbula, 

 Id. Gymnoblastic Hydroids, pp. 59-60, and Fig. 30. In Cladocarpus , the phylactocarp is a super- 

 added bifurcating branch, and in Pleurocarpa it consists of phylactocarpal appendages in the 

 shape of ribs taking the place of the proximal hydrocladia of a branch. 



3 See Allman, Gymnoblastic Hydroids, pp. 1 24-6 ; Id. Plumularidae, Challenger Reports, vii. 

 p. 4 ; Agassiz, Natural History of United States, iv. p. 267 ; Hamann, J. Z. xv. p. 30. 



* The Medusae Eleutheria dichotoma and Clavatella prolifera have so feebly developed a bell 

 that they are incapable of swimming, and only creep about. See Allman, op. cit. ante, pp. 31, 212, 

 384, and PI. XVIII ; Hincks, op. cit. i. p. 71 ; Haeckel, System der Medusen, pp. 105-7, and 

 Hartlaub, Z. A. ix. 1886. The Medusae in question appear to be distinct, not, as Haeckel con- 

 siders them, one and the same species : see Allman and Hincks. 



