SERTULARIA. 185 



§ 9. Sertularia (Plumularia) pinnata. — Plate XXXV. — This is a 

 delicate product, truly resembling a feather. It rises three inches, or little 

 more, in height, and is generally of a greenish colour. Slender twigs, with 

 a slight elegant recurvature, issue from each side of the stem in alternate 

 arrangement, shortening gradually as they ascend or descend from about 

 the middle of the specimen, and terminate the summit by a mere pro- 

 jection.— Plate XXXV. fig. 1. 



A row of low cells or denticles, somewhat apart, projects from the 

 convexity of the twig, which, in a certain position, would induce the ob- 

 •server, by some illusion, to ascribe a spine to the orifice. Each denticle 

 or cell is inhabited by a hydra, with about 20 muricate tentacula. When 

 the product is in maturity, there is only one hydra on the twig at the 

 summit, two on the next, then three, and so on to seven, beyond which 

 number I have not observed more. The extremity of no part is a hydra, 

 though it may be preparing from the evolution of one, for the mode of 

 increment seems to ensue from a twin bud ; the more mature of the twins 

 unfolding a perfect hydra, and the other advancing beyond it, to deve- 

 lope somewhat later. — Fig. 2. The cells are low, but quite conspicuous, 

 fig. 3 ; their tenants show nothing particular. The tentacula are rather 

 short and stout. — Figs. 4, 5. A waving pith occupies the stem. 



Numerous vesicles are huddled together on a portion of the stem, 

 presenting a reddish or yellowish appearance to the eye from their accu- 

 mulation. These vesicles are not of uniform figure, the edge of the orifice 

 of some being even, while two or three prongs extend that of others. — 

 Fig. 6. Each contains a single yellow or orange corpusculum, fig. 7, which 

 is discharged as a planula at various seasons of the year, from July and 

 August to December. — Figs. 8, 9. But all planulse do not seem invariably 

 of the same colour, as some produced in this last month, from what I con- 

 cluded the same species of Sertularia, were grey. 



The vesicles huddled together on the stem of a prolific specimen pro- 

 duced yellow planulse, extending about a third of a line, on August 12 and 

 13. Several crawled along; the motion of others tended to an orbit from 

 ])artial contraction of the body. — Fig. 9. One was monstrous. — Fig. 9, a. 

 Roots were diffiising from this monstrous planula in a week ; and in a few 

 VOL. L 2 a 



