38 Proceedings of the Royal Physical Society. 



mit of a slight eminence (PL I., Fig. 12), leads into the gastric 

 tube or stomodseum, the flattened lumen of which is much 

 encroached upon by thickenings of the walls. This tube opens 

 freely at its lower or aboral end into a body cavity, which is 

 limited by the base below, and by the body- walls at the sides, 

 while it is continued above into the cavities of the tentacles. 

 This body cavity is crossed by eight delicate membranes, the 

 mesenteries, which are attached to the outer wall of the 

 gastric tube at equidistant points (which alternate with the 

 tentacles above), and radiate outwards to be attached by their 

 outer edges to the inner surface of the body wall. 



Three layers enter into the constitution of the body: 1st, the 

 ectoderm covers the entire outer surface of the base and body 

 wall, both retractile and non-retractile parts, of the tentacles 

 and their pinnae, and is continued in at the mouth opening 

 to line the gastric tube in its entire extent ; 2d, the endoderm 

 lines the body cavity throughout, coats both lateral faces of 

 the eight mesenteries and the outer surface of the gastric 

 tube, and lines the inner surfaces of the tentacles; Sd, the 

 mesoderm lies in all parts of the body between ectoderm and 

 endoderm, consequently it enters into the composition of the 

 body wall, the tentacles, the gastric tube, and the mesenteries. 

 In some places the mesoderm becomes greatly thickened, and 

 in certain regions may develop calcareous spicules in its 

 interior. 



In a transverse section through the upper part of the body 

 of a retracted polype, such as is shown in PL I., Fig. 16, 

 one finds two concentric rings of tissue united hj eight 

 narrow radiating bands (see PL I., Fig. 16, my.). The outer 

 ring is the non-retractile body wall, formed of an outer layer 

 of ectoderm and an inner layer of endoderm, united by a thick 

 mass of mesoderm. The inner ring is the retractile or invasi- 

 nated body wall, and is formed by an outer layer of endoderm 

 and an inner layer of ectoderm united by thick mesoderm. 

 The lumen of the tube (^. t) formed by this invagination 

 is flattened from side to side, and then somewhat irregu- 

 larly pulled out radially towards the points of attachment of 

 the mesenteries which cross the body cavity, and are formed 

 by plates of mesoderm covered on both sides by endoderm, 



