iv PHYLUM CCELENTERATA 137 



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

 (vide infra). 



A great simplification of the colony is produced in Mi/riothela 

 (Fig. 93, 2) in which the short coenosarc 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 (in) 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. 24 and 94) consists of a simple cylindrical body 

 with a conical hypostome and a circlet of six or eight tentacles. 

 It is ordinarily attached, by yirtue 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. There are no distinct muscle-fibres, but the 

 large ectoderm cells are produced into muscle processes (C, in. pr) 

 which serve the same functions. There is no perisarc. Buds 

 (bd. 1, ltd. 2) are produced which develop into Hydree, 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 Protohydra (Fig. 95) and Microhydra, in 

 which the tentacles are absent. 



! 



FIG. 95. Protohydra leuckartii. (From Chun, after Greeff.) The mouth is to the left, the 



disc of attachment to the right. 



The polypes are usually cylindrical, as in Obelia, but in some 

 genera they are widened out into a vase-like form (Fig. 93, 5), in 

 others elongated into a spindle-shape (4). The tentacles may be 

 disposed in a single circlet, as in Obelia and Hydra, or there may 

 be an additional circlet round the hypostome (3, 5) or at the base of 

 the polype, or they may be scattered irregularly over the whole 

 surface (4). In Myriothela (2) they are short and so numerous 

 as to have the appearance of close-set papilla?. In some forms 

 they are knobbed at the ends, the knobs being loaded with stinging- 

 capsules (4). 



In some species a dimorphism of the hydranths obtains, some 

 of them being modified to form protective zooids. In Hydractinia 



