PLUMATELLA. 135 



3. It propagates by ova of lenticular form, with a smooth circumferential 



edge, which are liberated chiefly by decay in the season of autumn. 



4. The ovum contains a single embryo hydra, which reaches maturity 167 



days after being liberated from the Alcyonella. 



B. Alcyonella gclatinosa. 



1. Invests wood, sometimes by a thick, yellowish, or grey, stratum, from 



which prominences arise, as in the Lobularia or Alcyonium digitatum. 



2. The hydra, yellowish- grey, larger than that of the former, is provided 



with 70 tentacula. 



3. It propagates by ova of a lenticular form, with a smooth edge, which 



are generated amidst the stratum. 



4. They ascend into the body of the hydra, and many pass from the body of 



one into that of another. 



5. They are liberated in autumn, chiefly by decay of the hydrce, and of tin 



stratum ; but some may remain in channels of the wood during winter. 



6. The embryo attains maturity in T89 days from the date of liberation. 



7. The young of the Alcyonella, of the third or fourth generation, identi- 



cally resemble the parents of the first. 



III. Plumaiella. — A zoophyte, with tubular stem, boughs, and 

 branches, adhering laterally to subjacent substances. 



1 . A greenish hydra, with from 50 to 70 ciliated tentacula, protrudes from 



the extremities of the parts, and is retracted within. 



2. It propagates by ova, which are sometimes contained in the living 



hydra, but are seen in numerous rows in the tubular branches, whence 

 they are liberated in autumn, or earlier. 



3. An embryo hydra is contained in each ovum, which attains maturity m 



198 days. 



Among the general analogies of the three different products, the fol- 

 lowing are evident. 



I. The common conformation of the hydra in some of the essential 

 organs. 



II. In the decay of all with the lapse of a single season. 



