30 MORPHOLOGY OF INVERTEBRATE TYPES 



the same three layers as the body of the polyp. These layers 

 are ectoderm, mesoglcea and endoderm. The mesoglcea gives 

 the bulk to the body. It is a transparent, gelatinous, structure- 

 less substance containing much more water than organic matter. 

 It fills out the space between the ectoderm and the endoderm 

 and is not only found in the umbrella and peduncle, but forms 

 also a thin layer in the velum, manubrium and tentacles. The 

 endoderm forms the lining of the gastro-vascular system and 

 of the tentacles. The ectoderm forms the covering of the exum- 

 brella, subumbrella, peduncle, manubrium, tentacles and velum. 

 To the ectoderm belong further the gonads, the muscular 

 system and the nervous system with the sense organs. 



Endoderm. The endoderm of the medusa is composed 

 of the same elements as that of the polyp except that myo- 

 epithelial cells are absent. The lining of the stomach is 

 formed by a single layer of digestive endoderm and unicellular 

 glands; the lining of the canals is formed by circulatory 

 endoderm; and the lining of the tentacles by supporting 

 endoderm. 



Body covering. The covering of the exirmbrella consists of 

 a single layer of ectodermal flat epithelial cells. Near the margin 

 of the exumbrella nematocysts are found also between the flat 

 cells. The ectodermal layer of the subumbrella is more compli- 

 cated. It consists of two distinct layers, a superficial or epithelial 

 layer and a deeper or subepithelial layer. It must be borne in 

 mind, however, that the subepithelial layer has been differen- 

 tiated from the epithelial layer. The epithelial layer is composed 

 of indifferent columnal epithelial cells, myo-epithelial cells and 

 sensory cells. The latter, however, are very few in number. 

 The subepithelial layer is composed of ganglionic cells, nerve 

 fibres, striated and unstriated muscle fibres. The same elements 

 are found in the ectoderm of the peduncle and manumbrium. 

 There are, however, many sensory cells on the manubrium and 

 nematocysts on the lips. The ectoderm of the tentacles is com- 

 posed of indifferent cells, myo-epithelial cells, sensory cells and 



