50 MORPHOLOGY OF INVERTEBRATE TYPES 



lining of the gastro-vascular system, including the mesenteries, 

 the covering of the gullet, and the acontia. The gonads are also 

 endodermal. The epithelial layer of the endoderm consists of 

 columnar flagellated cells and glands as well as numerous long 

 nematocysts in the acontia. The subepithelial layer consists of 

 ganglionic cells, nerve fibres, muscular fibres and nematoblasts 

 in the acontia. (c) Mesogl&a. The mesoglcea is found be- 



FIG. ii. Nervous system of an actinia after Wolff. 



tween the two preceding layers in the wall, tentacles, peristome, 

 gullet and mesenteries. It is formed by a striated substance in 

 which multipolar connective cells are enclosed. 



Muscular system. The muscular system in Metridium 

 is highly developed. Both ecto- and endodermal muscles are 

 formed by my o- epithelial cells, but in many cases the fibre may 

 become independent of its mother cell. All ectodermal fibres 

 are longitudinal in position. The endodermal muscle fibres of 

 the wall and tentacles are circular. At the anterior end of the 



