138 METAZOAN PHYLA 



known as mesoglea. The outer cell layer is made up of smaller cells 

 varying in shape, though typically cubical, the outer ends of which form 

 a fairly even surface covered by a delicate cuticle. This layer is known 

 as the ectoderm and includes a number of different types of cells which 

 are scattered and not associated in such a way as to form tissues. Among 

 these cells are epitheliomuscular cells, from which arise contractile 

 fibers ; sensory and nerve cells, of which conducting fibers in the mesoglea 

 are branches; irregular interstitial cells; and on the basal disc, gland cells. 

 The inner cell layer, known as the entoderm, is composed of short columnar 

 cells of larger size and more irregular shape. The surface of the entoderm 

 is neither so even as is that of the ectoderm nor does it possess a cuticle. 

 In this layer are epitheliomuscular cells, nerve cells, and gastric gland 

 cells. In the tentacles the cells of both layers become much shorter and 

 the whole wall very much thinner. 



The mesoglea, which is the layer between these two cell layers, 

 is composed of several elements: (1) a supporting lamella, jelly-like in 

 consistency, secreted by the cells of the two other layers, and giving a 

 certain degree of support to the body; (2) two networks of nerve fibers, 

 derived from the nerve cells in the two cell layers; (3) contractile fibers, 

 prolongations of the epitheliomuscular cells in both ectoderm and 

 entoderm. 



164. Nematocysts. — Interstitial cells in the ectoderm of the body 

 in which are formed nematocysts are known as cnidohlasts. A nematocyst 

 (Fig. 59) consists of a sac of fluid within which is coiled a thread. When 

 fully developed, the cnidoblast also possesses a projecting, sharply 

 pointed cnidocil. As the cnidoblasts approach full development they 

 migrate to the surface, and the projecting cnidocil, when it is stimulated, 

 causes the nematocyst to react. In this reaction the coiled thread is 

 thrown out, probably as a result of increased pressure within the sac. 

 In one kind of nematocyst this thread is hollow, sharply barbed at its 

 base, and carries a poison which serves to anaesthetize the animal into 

 which it is discharged (Fig. 59, A and B). In another type the thread 

 is barbless, elastic, and becomes coiled around the object against which 

 it is discharged (Fig. 59 E). By thus coiling around the spines and 

 hairs of the prey these nematocysts impede its movements. No cnido- 

 blasts seem to originate on the tentacles themselves, but great numbers 

 migrate from the place of origin in the body to the tentacles, where the 

 nematocysts can be used most effectively in the capture of food. 



165. Neuromuscular Mechanism. — While the hydra cannot be said 

 to possess tissues, much less organs and systems, there are developed 

 definite mechanisms out of the variety of cells which the body possesses. 

 Among these is the neuromuscular mechanism. This is made up of the 

 scattered sensory cells lying on the surface of both ectoderm and ento- 



