378 Verrill, JSFotes on Radiata. 



Order, ALCYONARIA. 



Sub-Order, PENNATULACEA. 



Family, Renillid^e. 



Renilla. 



The polyps arise from the upper surface of a flat, reniform, cavernous 

 disk or frond, having a sinus on one edge, near which there is upon 

 the lower surface a locomotive peduncle, which is muscular and 

 greatly extensible and divided in the interior into two longitudinal 

 chambers, which communicate with two large cavities at its base, and 

 through these with the smaller cavities of the disk, and thus with the 

 bodies of the polyps. The integument of the lower surface, peduncle, 

 and upper surface, is filled with numerous, slender, prismatic spicula, 

 and around the bases of the polyps there are pointed, projecting groups 

 of similar spicula. The polyps originate by budding around the 

 edo'e of the disk, and are therefore regularly arranged, alternately 

 both in consecutive circles and in radiating lines, which are symmet- 

 rical upon the right and left side of a median plane passing through 

 the sinus, and they are smaller and more crowded toward the edge 

 than on the central parts. The polyps are rather large, much exsertin 

 expansion, but wholly retractile. 



Besides the ordinary form of polyps, there are in this, as in other 

 o-enera of Pennatulacea, a second kind, having a different structure 

 and appearance. Or, in other words, the polyps are dimorphous in 

 a manner analogous to that observed in many Hydroids. In Renilla, 

 the second kind of polyps are scattered thickly over the upper 

 surface between the others, and appear in alcoholic specimens like lit- 

 tle papilla?, with clusters of whitish spots on their surface, and sur- 

 roimded with spicula similar to those around the ordinary polyps, but 

 less numerous and smaller. They are also asexual. 



The writer first described these peculiar dimorphous forms of the 

 polyps of Renilla, in 1864,* as " rudimentary polyps," and afterwards 

 those of Leioptillum undulatum, Ptilosarcus Gurneyi, Veretillum 

 Stinijjsonu, etcf 



* Revision of the Polyps of the Eastern Coast of the United States, Memoirs of the 

 Boston Society of Natural History, vol. i, p. 12. 



f Proceedings of the Essex Institute, vol. iv, p. 182-5, 1865. 



