COELENTERATA 51 



termed the hydrorhiza, and the single stem the hydrocaulus. 

 Both the hydrorhiza and the hydrocaulus are protected by 

 a thin chitinoid investmont, the joerisarc. 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 of 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 iutracellularly. 

 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 



