PHYLUM CCELENTERATA 109 



length. They may be likened to a short, thick thread unraveled 

 at the unattached, distal end. 



Morphology. - - EXTERNAL FEATURES. - - The body of Hydra 

 is really a tube usually attached by a basal disk at one end, and 

 with a mouth opening at the distal or free end. Around the 

 mouth are arranged from six to ten smaller tubes, closed at their 

 outer end, called tentacles (Fig. 65, /). Both the body and ten- 

 tacles vary at different times in length and thickness. One or 

 more buds (Fig. 65, b) are often found extending out from the 

 body, and' in September and October reproductive organs may 

 also appear. The male organs (testes, Fig. 65, y.t, m.t) are con- 

 ical elevations on the distal third of the body; the female organs 

 (ovaries, Fig. 65, y.e, m.e) are knoblike projections near the 

 basal disc. 



STRUCTURE (Fig. 65).-- Hydra is a diploblastic animal con- 

 sisting of two cellular layers, an outer thin, colorless layer, the 

 ectoderm (ec.) and an inner layer, the entoderm (en.}, twice as 

 thick as the outer, and containing the brown bodies which give 

 Hydra fusca its characteristic color. Both layers are composed 

 of epithelial cells. A thin space containing a non-cellular jelly- 

 like substance, the mesoglea (mes.}, separates ectoderm from 

 entoderm. Not only the body-wall, but also the tentacles, pos- 

 sess these three definite regions. The body, with the exception 

 of the basal disc, is covered by a thin, transparent cuticle. Both 

 body and tentacles are hollow, the single central space being 

 known as the gastrovascular cavity (gv.c.). 



The ectoderm is primarily protective and sensory, and is made 

 up of two principal kinds of cells: (i) epit/ieliomuscular, and (2) 

 interstitial. The former are shaped like inverted cones, and pos- 

 sess long (up to .38 mm.), unstriped contractile fibrils at their 

 inner ends; these enable the animal to expand and contract. 



The interstitial cells lie among the bases of the epitheliomuscular 

 cells; they give rise to three kinds of nematocysts or stinging 

 cells (Fig. 65, n; Fig. 66). Nematocysts are present on all parts 

 of the body except the basal disc, being most numerous on the 



