148 ZOOLOGY SECT. 



Hydractinia belongs to the Anthomedusae : the Leptomedusan 

 Claihrozoon, an Australian genus, resembles it in having branched 

 jand intertwined coenosarcal tubes, the perisarc of which under- 

 Igoes fusion ; but the complex mass thus produced, instead of 

 forming an incrustation on a shell, is a large, abundantly branched, 

 tree-like structure, resembling some of the fan-corals or Gorgonacea 

 (vide p. 201). Ceratella (Fig. 107) has a similar fan-coral-like 

 appearance, with a branching axis composed of numerous inter- 

 twining and anastomosing tubes ; but while Clathrozoon possesses 

 thecae, in Ceratella they are absent. 



A great simplification of the colony is produced in Myriothela 

 (Fig. 106, 2), in which the short ccenosarc bears a single large 

 terminal hydranth, and gives off numerous slender branches which 

 bear the reproductive zooids (s). Even greater simplicity is found 

 in Corymorpha (3), in which the entire organism consists of a 

 single-stalked polype, from the tentacular region of which the 

 medusae (m) arise. 



But the simplest members of the whole class, with the exception 

 of one or two imperfectly known forms which will be referred to 

 below, are the Fresh-water Polypes of the genus Hydra. The 

 entire organism (Figs. 27 and 108) consists of a simple cylindrical 



body with a 

 conical hypo- 

 stome and a 

 circlet of six 



FIG. 109. Protohydra leuckartii. (From .Chun, after Greeff.) i , . ^ 



The mouth is to the left, the disc of attachment to the right. eignt 



tacles. It is 



ordinarily attached, by virtue of a sticky secretion from the 

 proximal end, to weeds, &c., but is capable of detaching itself 

 and moving from place to place after the manner of a looping 

 caterpillar. The tentacles are hollow, and communicate freely 

 with the enteron. Both the body and the tentacles are highly 

 contractile, the contractions being effected by means of a layer of 

 fibres which run longitudinally. These fibres are processes the 

 muscle processes (C, m. pr.) of the large ectoderm cells. Similar 

 shorter muscle processes of some of the endoderm cells run 

 circularly and antagonise the longitudinal fibres. Nematocysts are 

 abundant in the ectoderm. The endoderm cells are mostly 

 amoeboid and vacuolated. Each usually bears one or more flagella, 

 but these may be retracted. Glandular cells occur here and there. 

 Nerve-cells (multipolar) occur in both layers, but present no regular 

 arrangement. There is no perisarc. Buds (bd. 1, bd. 2) are 

 produced which develop into Hydras, but these are always detached 

 sooner or later, so that a permanent colony is never formed. There 

 are no special reproductive zooids, but simple ovaries (ovy) and 

 testes (spy) are developed, the former at the proximal, the latter 

 at the distal end of the body. Even simpler than Hydra are 



