48 MORPHOLOGY OF INVERTEBRATE TYPES 



and exocoeles communicate with each other below the gullet, 

 where the inner edge of the primary mesenteries is free and where 

 all chambers open into the central cavity. The chambers com- 

 municate with each other also at the anterior end. This com- 

 munication is established by means of large round or oval open- 

 ings in the mesenteries. The openings are arranged in two 

 circles; one surrounding the gullet close under the inner surface 

 of the peristome, the other somewhat lower and close to the wall. 

 There are therefore altogether twenty-four openings, two in 

 each mesentery. These openings are called inner and outer 

 mesenteric ostia or septostomata. The free edge of the primary 

 mesenteries forms a thickened and twisted mesenteric filament. 

 Near the base of the mesentery the filament becomes free, 

 changes somewhat its structure and appears in the shape of a 

 long and thin thread or acontium. Each exoccele is subdivided 

 by incomplete mesenteries, which are attached only to the wall of 

 the polyp but not to the gullet and which have only outer septo- 

 stomata. There are usually one pair of secondary, two pairs of 

 tertiary, and four pairs of quaternary incomplete mesenteries in 

 each exoccele, or altogether forty- two pairs of incomplete mesen- 

 teries in the six exocoeles. Each incomplete mesentery has a 

 mesenteric filament along its free edge and an acontium at the 

 base. In the wall of the incomplete mesenteries the gonads are 

 situated and appear as a single row of bead-like bodies. When 

 fully developed they fill almost completely the exocoeles. Small 

 gonads develop also near the free edge of the primary mesen- 

 teries, except the directives, below the gullet. 



Microscopic anatomy, (a) Ectoderm. The ectoderm forms 

 the covering of the body, of the tentacles, and of the peristome, 

 and the lining of the gullet. Although composed of a single layer 

 of cells it may be divided into an epithelial and a subepithelial 

 layer. The former consists of ciliated columnar cells, mucous 

 glands, albuminous glands, nematocysts, and sensory cells; the 

 latter of nematoblasts, ganglionic cells, nerve fibres, muscle cells 

 and muscle fibres, (b) Endoderm. The endoderm forms the 



