68 CLASS II. 
set in rhomboidal rows: then comes a second stomach ending below 
ina blind sac that is continued at the upper part into an intestine, 
which ascends by the side of the stomach and cesophagus, and 
near the mouth, or a little below it, terminates in the anus. 
Brown follicles cover the external wall of the stomach, and seem to 
represent the liver! In some Polyps a circulating system has 
been observed, or at least vessels, which probably arise as branches 
from the intestinal tube, anastomose with one another in the Polyp- 
stock, and effect a communication between the different individuals’. 
In many, moreover, a stream of water is found, which penetrates 
by the mouth into the canals of the Polypary. It has been observed 
that the flow is caused by cilia on the walls of the canals. Probably 
this motion is in connexion with the function of Respiration. 
LIsTER saw in the stem of Plumularia pluma Lam. the stream in 
the same canal moving alternately in opposite directions ®. 
We have seen above that propagation in Polyps is usually 
effected by buds. In Hydra, after bemg developed, they are sepa- 
rated: in others they remain attached to the parent-stem. But 
besides this mode of propagation, a sexual generation has been 
observed in this class. In Hydra, about winter-time, a periodical 
development of eggs on the inferior part of the body has been ob- 
served. The thm membrane, surrounding the egg as it projects 
from the body, bursts, and the egg attaches itself to some object or 
other in the water. In some species the yolk-membrane is covered 
with cloven ramiform processes, as if with spines. After two or 
three months the young one is visible. The conical excrescences 
which arise higher up on the body at the base of the arms, and 
which are perforated at the point, contain spermatozoa ; and may, to 
a certain extent, be considered to be external testes*. These genital 
1 See A. Farr, Observations on the minute structure of the higher forms of Polypi. 
Phil. Trans. 1837, pp. 387—426. Pl. XX—xxXvII. 
2 Minne Epwarps, Ann. des Sc. nat. sec. Série, 1v. Zool. p. 338. 
3 Phil. Trans. 1834, p. 369. 
4 The egg of Hydra was figured long ago by RoEsEL, Suppl. Tab. LXxxul. fig. 1a 
and fig. 2. See also the figures of EHRENBERG and ERDL in WAGNER, Icon. Zootom. 
Tab, xxxtv. figs. 8 and 10, and of Laurent, Recherches sur ’ Hydre et V Eponge deau 
douce, Paris (1844), Pl. 1. Here figs. g—14 the exclusion from the egg is figured, 
which had also been observed by Pallas: ‘‘ Ovula autumno generare Hydras observatum 
est... polypi compendium per hyemem duraturum continentia.—Hane per ovula propa- 
gationem bis mets oculis perfectam observavi.” Elench. Zoophytor. p. 28. 
