158 THE INVERTEBRATA 



of the colony. The ectoderm, mesogloea and endoderm of the polyps 

 are of course continuous with the same layers in the coenosarc of the 

 colony, but while the ectoderm is only a thin skin composed of a single 

 layer of cells spread over the surface of the whole colony, the meso- 

 gloea is expanded to form a bulky mass of jelly which is traversed by 

 the endodermal tubes of the polyps. These run parallel with each 

 other without joining for considerable distances, but they are con- 

 nected by other endodermal tubes which are much more slender, so 

 that, like a hydroid colony, the alcyonarian colony has a common 

 coelenteric system. 



Fig. 132. Diagram of section through colony of Alcyonium showing ex- 

 tended polyps with pinnate tentacles and coenosarc. Original. The direction 

 of water-circulation is shown by arrows. The mesogloea is indicated by dots 

 and the spicules it contains by small crosses. D, dorsal and V, ventral sides 

 of polyp ; ect. ectoderm ; end. endoderm ; sol. solenia and end.s, solid endoderm 

 strands ; bd. endodermal bud which will give rise to a new polyp ; mes.d. the 

 two dorsal mesenteries; mes.' the other mesenteries; std. stomodaeum. 



The polyps are delicate and withdraw on the slightest stimulus, the 

 oral disc with its crown of tentacles being pulled inside the enteron 

 by the contraction of longitudinal muscles running in the mesenteries 

 and attached to the oral disc. By a continuation of this contraction 

 the whole column of the polyp is introverted ("turned outside in", 

 as with the finger of a glove). This is the condition in which preserved 

 colonies oi Alcyonium are nearly always found, and tangential sections 



