THE CGELENTERATA 151 



only two cellular layers, ectoderm and entoderm, is designated two- 

 layered or diploblastic, and is characteristic of ccelenterates. In 

 order to examine the structure of the layers more fully we may 

 either crush an individual under a cover slip by means of a tap with a 

 pencil, or, better still, immerse it for a short while in a macerating 

 fluid.* 



We have already seen that the ectoderm is composed of two 

 groups of cells, those forming the main epithelium and those in the 

 interstices, and we can now studv them more in detail. The larger 



N.D. 



FIG. 48. Diagram of a transverse section of part of body wall of Hydra. 



E., entoderm ; EC., ectoderm ; I., Interstitial cells ; M., mesoglea ; N., nematocyst ; 

 N.D., discharged nematocyst ; V., vacuole ; Z., zoochlorella?. 



cells are known as the epithelio-muscular cells, and are roughly cone- 

 shaped. Their outer sides, i.e. the bases, are very firmly joined 

 together, so that it is a matter of difficulty to isolate them save in 

 groups, and they are, moreover, covered by a thin homogeneous 

 cuticle. Their inner ends, the apices, are blunt and give off fairly 

 long processes, running transversely to the long axis of the cell. 

 These protoplasmic filaments are the portions of the cell especially 

 set aside for contraction, and hence distinguished as the muscle 

 processes. They lie embedded in the mesoglea, and careful examina- 

 tion reveals the presence in them of minute myofibrillee, the special- 

 ised contractile elements. Thus the ectoderm furnishes Hydra with 

 an external layer of longitudinally running muscles, by the contraction 

 of which the shortening of the body is brought about. The body of 



* E.g. Schneider's fluid, one part of '02 % solution of osmic acid and four 

 parts of 5 % solution of acetic acid mixed together. The animal requires to 

 be immersed from three to five minutes and then handled with care, for it 

 disintegrates readily. 



