COELENTERATA 51 
termed the hydrorhiza, and the single stem the hydrocaulus. 
Both the hydrorhiza and the hydrocaulus are protected by 
a thin chitinoid investment, the pervsare. This tubular 
covering, however, stops short at the base of each hydranth. 
The living tissue within the perisarc is known as the coenosarc ; 
it consists externally of a single layer of ectoderm, which 
secretes the perisarc, and internally cf an axis of endoderm 
traversed by ciliated canals; these serve to place the cavities 
of the various hydranths in communication with one 
another. 
Each hydranth is a somewhat flask - shaped structure 
bearing two rings of tentacles; one composed of short tentacles 
situated round the mouth, the other, of much larger tentacles, 
arises from the middle of the body, where the diameter is 
greatest. The bases of the circumoral tentacles are visible as 
slight projections on the body-wall for some little distance below 
their point of emergence. Their ectoderm is crowded with 
nematocysts, and contains muscle cells, glandular cells, ganglion 
cells, usually bipolar, and four kinds of nematocysts. Their 
endoderm consists of several rows of cartilaginoid cells, which 
serve as a stiffening skeleton. 
The digestive cavity of the hydranth is spacious, it is lined 
by endoderm cells, which in many species digest intracellularly. 
There is a thickened layer of endoderm forming a ring for the 
support of the larger tentacles. The middle layer or mesogloea 
is structureless and thin. 
The relationship of the hydroid to the medusoid person is 
best explained by means of diagrams (Fig. 39). The oral axis 
of the hydroid is shortened, and the circumference of the middle 
part of the body is correspondingly increased. The ring of large 
tentacles is thus carried out to the edge of what is known as 
the umbrella ; the oral cone, with its ring of tentacles, remaining 
in the position of the umbrella handle. This change of external 
shape is accompanied by the obliteration of certain parts of the 
coelenteron or central cavity. Along certain areas in the 
umbrella the endoderm cells come in contact and fuse, forming 
the endodermal lamella (Fig. 39, II. 12). This fusion takes 
place in such a way as to leave certain tracts open, the most 
important of which are the circular vessel running round the 
