J62 MANUAL OF ELEMENTARY ZOOLOGY 



upon an oral cone or hypostome ; it leads into the hollow of 

 the cylinder, with which the hollows of the tentacles are 

 continuous. This space is the enteron. The cylinder is 

 rather wider in the middle than near the ends. The wall of 

 the body is composed of two protoplasmic layers, the outer 

 known as the ectoderm and the inner as the endoderm, with 



a structureless la- 

 mella or mesoglea 

 between them, con- 

 sisting of a gelatin- 

 ous substance which 

 they secrete. Such 

 a body as this is 

 known as a polyp. 

 The ectoderm 



Ectoderm. C ? nsistS 



of sev- 

 eral kinds of cells, of 

 which the most con- 

 spicuous are those 

 known as musculo- 

 epithelial cells. 

 These are roughly 

 conical in shape, 

 with their broad 

 ends directed out- 

 wards and fused to 

 form a continuous 

 Fjg. ioi.-Two specimens of Hydra magnified, la y er of P^toplasm 

 one contracted, the other in a state of 0Ver the body, 

 moderate expansion, the latter bearing two The apex of the 

 buds in different stages. cone is drawn out 



m., Mouth; or.c, oral cone. into One Or more 



contractile processes, 

 which run along the cylinder and tentacles, at right angles 

 to the main part of the cell, forming a distinct layer on 

 the outer side of the structureless lamella. Over the 

 greater part of the body the surface layer of the proto- 

 plasm is a firm pellicle, but in the disc this is absent. The 

 cells in this region are also peculiar in containing granules of a 

 substance secreted by the protoplasm which is used to fix the 



