68 CLASS II. 



set in rhomboidal rows : then comes a second stomach ending below 

 in a blind sac that is continued at the upper part into an intestine, 

 which ascends by the side of the stomach and oesophagus, 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 1 . 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 2 . 

 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 3 . 



We have seen above that propagation in Polyps is usually 

 effected by buds. In Hydra, after being 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 thin 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 4 . These genital 



1 See A. FARRB, Observations on the minute structure of the higher forms of Polypi. 

 Phil. Trans. 1837, pp. 387 426. PI. xx xxvii. 



2 MILNE EDWARDS, Ann. des Sc. nat. sec. Se'rie, iv. Zool. p. 338. 



3 Phil. Trans. 1834, p. 369. 



4 The egg of Hydra was figured long ago by ROESEL, Suppl. Tab. LXXXIII. fig. i a 

 and fig. 2. See also the figures of EHRENBERG and ERDL in WAGNER, Icon. Zootom. 

 Tab. xxxiv. figs. 8 and 10, and of LAURENT, Recherches sur VHydre et VEponge d'eau 

 douce, Paris (1844), PI. n. Here figs. 9 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 duraturuni continentia. Hanc per ovula propa- 

 gationem 6w meis oculis perfectam observavi," Blench. Zoophytor. p. 28. 



