MISCELLANEOUS. 57 



§ 2. Sertularia thuia. — Bottle brush. — Plate XIX. Figs. 1-6. — 

 Instead of reserving a few remarks on this zoophyte for an appendix, as 

 the more suitable place, they are inserted here. 



The peculiar structure of the product has attracted the attention of 

 most naturalists. 



It is almost always seen as a knotted stalk ; as if crowned by an en- 

 larged head, the summit of which is 12 inches above the root in the finest 

 specimens. 



The knots merely indicate the places which have lost former branches, 

 and the enlarged head is composed of those still subsisting. Their ar- 

 rangement is alternate on the stalk, and they subdivide dichotomously, the 

 subordinate parts bearing a double row of cells. 



These are occupied by a hydra with 12 or more muricate tentacula. 

 But to judge by the difficulty of obtaining living specimens, this must be 

 a very delicate animal ; insomuch that I have found it not only of the 

 utmost rarity, shewing itself transiently, nor surviving long. 



Prolific specimens bear numerous vesicles, crowded together on the 

 branches, which, at first sight, would be supposed the spawn of some ani- 

 mal confusedly deposited there. These vesicles generally contain one pure 

 white spherical corpusculum, sometimes two. In the latter case, one 

 side of each of the corpuscula is flattened, and the flat sides being next to 

 each other, a slight interval appears to separate them. 



Specimens laden with vesicles having been obtained in December, 

 a number of extremely small white globules, which relaxed into planula?, 

 appeared from them on the 25th of that month. And on January ] 1, 

 above 100 minute white planulse were in the vessel along with the others. 



In another week the planul* in different vessels amounted to hun- 

 dreds. They exhibited no peculiarities. Many had fallen into a watch- 

 glass adapted for their reception, but nearly all were lost by supervening 

 decomposition. The observer has not the choice of seasons, for he must 

 avail himself of opportunities. Summer, however, or the prevalence of 

 genial temperature, is the more likely to aid his enquiries, for then the 

 advances of every thing to maturity are more speedy. 



VOL. II. h 



