TYPE CCELENTERA. 77 



iBg at its extremity the mouth-opening. This leads into 

 a central cavity, the coelenteron, lying in the substance of 

 the bell, and from this pouches or fine canals radiate out 

 towards the rim, where, in some cases, they are united by a 

 circular canal which runs completely round the bell at this 

 region. To the margin of the bell tentacles are usually 

 attached, and sense-organs, presenting frequently consider- 

 able complexity of structure, are found in the intervals be- 

 tween the tentacles or at their bases. These forms, known 

 as medusce, are, as a rule, free-swimming, propelling them- 

 selves through the water by vigorous contractions of the bell. 

 All Cnidaria, whether of the polyp or medusa form, pos- 

 sess in their tissues peculiar elements altogether unrepre- 

 sented in the Sponges. These are the 

 nematocysts or so-called thread-cells. 

 Each consists of an oval or spherical 

 cyst (Fig. 38, c) with a membranous wall 

 and fluid contents, the wall being pro- * "^ 

 longed at one end into a long, exceed- 

 ingly delicate, hollow, thread-like fila- 

 ment sometimes furnished with spines at 

 its base, and, in . an undisturbed cyst, is 

 invaginated, and coiled up in the interior 

 (Fig. 38, t). These nematocysts are pro- 

 duced by and enclosed within special Fig. 38. — Nematoctst 

 cells known as cnidoblasts (en) lying Cell op Physalia. 

 principally in the ectoderm, and in their ^^ ^ °^™* "j'^^'' 

 most highly developed form differen- ^n = cnidoblast. 

 tiating below into a supporting stalk (s) n = nerve prolongation. 

 which rests upon the outer surface of « = supporting process, 

 the mesogloea. From the outer extremi- ~ "^^ 

 ty there projects beyond the surface of the ectoderm a short 

 hair-like process, the cnidocil (cT), and, in addition, an exceeding- 

 ly fine process of some length (n) is given off at the junction of 

 the stalk with the cyst-containing portion of the cell. These 

 two processes are supposed to be sensitive, the longer one per- 

 haps bringing the cnidoblast into connection with nerve-cells 

 lying elsewhere in the ectoderm. In some way not yet prop- 

 erly understood, a stimulation, such as a touch by some foreign 



