I04 Natural History Bulletin. 



it will simplify matters to carefully master the diagrammatic 

 figure, PI. Ill, fig. 6. I'he mesenteries are vertical plates con- 

 necting the gullet wall with the body wall. They consist es- 

 sentially of a plate of mesoderm connecting the mesoderm of 

 gullet with that of body. The eight mesenteries enclose eight 

 ^ spaces called inter-mesenterial spaces which open freely into 

 the general body cavity below and are each continuous with 

 the cavity of a tentacle above. It will be seen lh;it each inter- 

 mesenterial chamber is lined with endodermai cells. This 

 ■ endodermai lining extends upward into each teniacle where 

 the cells assume a somewhat columnar shape. (PI. I, fig. 8, d.) 



Returning to PI. Ill, fig. 6, w^e see that the gullet is the only 

 ectodermal structure found in the cross section. This is due 

 to the invagination by which the gullet is formed. 



Muscular System. By far the most important muscles 

 in the polyp are the retractor muscles which are in the 

 form of thin plates applied to the ventral side of each 

 mesodermal plate of the mesenteries and thus are situated 

 directly under the endodermai covering of the mesenteries. 

 These muscles are attached to the gullet along its entire 

 length, and extend below it into the body cavity where one 

 edge of the muscle plate is free, and the other is attached 

 to the lining of the calicle which offers resistance against 

 which the muscles act in retracting the polyp. 



In the expanded polyp, these muscles can be seen showing 

 through on the sideS of the mesenteries and extending upward 

 to between the bases of the tentacljs, where each muscle 

 divides into two parts which extend along the adjacent sides 

 of two tentacles, between the bases of which they fork. Each 

 of these branches constitutes a tentacular muscle. This latter, 

 then, is continuous with the retractor and each retractor 

 divides to form two tentacular muscles which extend along 

 the adjacent sides of two tentacles. There are therefore, six- 

 teen tentacular muscles, two to each tentacle, one on each 

 side. It will also be seen that each tentacle is acted upon by 

 two retractor muscles by which it is pulled down into an in- 

 termesenterial chamber durino; retraction. 



