TYPE CCELENTERA. 



109 



being united by transverse plates, and in Heliopora the skeleton 

 becomes very massive, resembling that of the ordinary corals 

 even to the occurrence of septa projecting into the interior of 

 the cups which contain the individual polyps. 



Notwithstanding these manifold variations of the skeleton 

 and of the colony form, the individual polyps present through- 

 out a great similarity of structure. They possess only eight 

 pinnate tentacles and eight mesenteries whose retractor mus- 

 cles are arranged in the manner shown in the annexed dia- 

 grammatic cross-section of a polyp (Fig. 61). In enitta and 



rm f / - 



FIG. 60. DIAGRAM OP YOUNG 

 COLONY OF Alcyonium (after 

 VON KOCH. 



FIG. 61. DIAGRAMMATIC TRANSVERSE 

 SECTION OF AN ALCYONARIAN. 

 rm = retractor muscle. 



si = siphonoglyphe. 

 I-IV mesenteries. 



allied forms, such as Pennatula, a slight polymorphism occurs, 

 certain polyps possessing no tentacles and functioning as 

 inhalent zooids through which currents of water pass into the 

 coelenteric cavities of the colony through which they circulate. 



2. Order Edwardsiae. 



The Edwardsiae never produce colonies nor do they possess 

 a skeleton, though frequently the exte ior of the body is en- 

 crusted by foreign particles. They live usually imbedded in 

 sand, the base being rounded and not adhesive, and possess 

 eight (sometimes sixteen or thirty-two) simple tentacles and 

 eight mesenteries, differing from those of the Alcyonarians in 



