l6o THE INVERTEBRATA 



in Fig. 132, mes.d.y and their persistence throughout the endodermal 

 tubes is necessary for the maintenance of the exhalant current. In 

 contrast with this the remaining six mesenteries have rounded fila- 

 ments covered with an epithehum consisting largely of gland cells. 

 Also at certain seasons the germ cells arise near the free border 

 (Fig. 136 F). Small organisms caught by the tentacles and introduced 

 into the enteron are embraced by these mesenteric filaments and held 

 fast while the fluid from the glands brings about a disintegration and 

 partial digestion of the tissues. Solid fragments of food resulting 

 from this are ingested by individual endodermal cells and the diges- 

 tion completed. Not only do the dorsal mesenteric filaments differ 

 from the others in function but they are ectodermal while all the rest 

 are endodermal. 



The mesogloea of Alcyonium is invaded by cells from the ectoderm 

 which form in their cytoplasm aggregations of calcium carbonate 

 with a characteristic shape which are called spicules. As the spicules 

 develop the secretory cells migrate into the deeper parts of the colony. 

 They are present in such numbers as to give a certain quality of 

 solidity to the colony, and on its death the spicules it contains remain 

 behind as a not inconsiderable mass. The part which alcyonarians 

 consequently play in the formation of coral reefs, though secondary, 

 is not unimportant. The mesogloea, as has been mentioned above, is 

 traversed by hollow strands of endoderm {solenia) which communicate 

 between the polyp tubes and also by solid endodermal strands which 

 may play some part in the secretion of the jelly of the mesogloea. 

 From the solenia, where they approach the surface, small buds are 

 formed which develop into new polyps. 



The gonads are developed from groups of endodermal cells near 

 the filaments, but they only occur on the six ventral mesenteries. The 

 eggs are comparatively large and pass very slowly up the enteron and 

 out of the stomodaeum, being fertilized outside the polyp and de- 

 veloping into a planula larva. After a free-swimming period this fixes 

 and becomes a single individual which by budding gives rise to a 

 colony. 



Variation in the Alcyonaria occurs mostly in the method of forma- 

 tion of the colonies and the skeleton. The simplest form is found in 

 Cornularia and Clavularia. From the original polyp a creeping stolon 

 with a single endodermal tube is given off, and this gives rise at in- 

 tervals to polyp buds, which may in turn produce fresh stolons. The 

 coenosarc of the colony thus forms a network like a hydroid colony. 

 In Alcyonium y as already described, the elongated polyps are crowded 

 together in bundles and fused along nearly the whole of their length, 

 the ectoderm and mesogloea of adjacent polyps being continuous, and 

 the endodermal tubes in frequent communication. The mesogloea 



