THE NEUEOMUSCULAE STEUCTUEE 79 



of muscles and are called directive mesenteries, the others 

 being designated as non-directives. Besides the pairs of 

 complete mesenteries, there are many other pairs of in- 

 complete ones. These are characterized by the fact that 

 they extend only a short distance into the digestive cavity 

 and thus fail to reach the oesophagus. 



The walls of a sea-anemone's body are in all places 

 relatively thin. They are for the most part epithelial in 

 nature and are composed of two layers of cells separated 

 by a third layer of partly secreted, partly cellular ma- 

 terial. Covering the whole exterior of the animal is the 

 ectodermic layer, which at the mouth is reflected inward 

 over the inner surface of the resophagus to the inner ter- 

 mination of that tube. The whole interior of the sea- 

 anemone is covered with the entodermic layer, which 

 unites with the ectoderm at the inner end of the esoph- 

 agus. Between the ectoderm, which thus covers the ex- 

 terior and the entoderm that lines the interior, is a third 

 layer, the supporting lamella. As has already been 

 stated this is composed of secreted substance containing 

 numerous cells. It is resistent enough to give a good deal 

 of support to the sea-anemone; hence it partakes of the 

 nature of a skeleton. The body wall of the sea-anemone, 

 as already mentioned, is everywhere relatively thin and 

 wherever it is punctured the three layers mentioned are 

 cut through, the ectoderm first, then the supporting la- 

 mella, and finally the entoderm, after which the digestive 

 cavity is invaded. 



The resolution of these layers of tissues into their 

 cellular elements was first successfully accomplished by 

 0. and E. Hertwig (1879-1880). Notwithstanding the dif- 

 ferences in position and function of the ectoderm and en- 

 toderm, there is much uniformity in their neuromuscular 



