160 THE MICROSCOPE AND ITS REVELATIONS. 



external integument, into which the soft membranous lining prolongs 

 itself; the cavity thus formed, however, is not to become (as in Hydra 

 and its allies) the stomach of the new zooid; but it constitutes the cham- 

 ber surrounding the digestive viscera, which organs have their origin in 

 a thickening of the lining membrane, that projects from one side of the 

 cavity into its interior, and gradually shapes itself into the alimentary 

 canal with its tentacular appendages. Of the production of gemmae 

 from the polypides themselves, the best examples are furnished by the 

 FlustrcB and their allies. From a single cell of the Flustra, five such 

 buds may be sent-off, which develop themselves into new polypides 

 around it; and these, in their turn, produce buds from their unattached 

 margins, so as rapidly to augment the number of cells. To this extension 

 there seems no definite limit; and it often happens that the cells in the 

 central portion of the leaf like expansion of a Flustra are devoid of con- 

 tents and have lost their vitality, whilst the edges are in a state of active 

 growth. Independently of their propagation by gemmation, the Polyzoa 

 have a true sexual generation; the sexes, however, being usually, if not 

 invariably, united in the same polypides. The sperm-cells are developed 

 in a glandular body, the testis m, which lies beneath the base of the 

 stomach; when mature they rupture, and set free the spermatozoa q q, 

 swim freely in the liquid of the visceral cavity. The ova, on the other 

 hand, are formed in an ovarium n, which is lodged in the membrane 

 lining the tegumentary sheath near its outlet; the ova, having escaped 

 from this into the visceral cavity, as at 0, are fertilized by the spermato- 

 zoa which they there meet with ; and are finally discharged by an outlet 

 at j9, beneath the tentacular circle. 



551. These creatures possess a considerable number of muscles, by 

 which their bodies may be projected from their sheaths, or drawn within 

 them; of these muscles, r, s, t, u, v, w, x, the direction and points of attach- 

 ment sufficiently indicate the uses; they are for the most part retractors, 

 serving to draw-in and double up the body, to fold-together the circle 

 of tentacula, and to close the aperture of the sheath, when the animal 

 has been completely withdrawn into its interior. The projection and 

 expansion of the animal, on the contrary, appear to be chiefly accom- 

 plished by a general pressure upon the sheath, which will tend to force- 

 out all that can be expelled from it. The tentacles themselves are 

 furnished with distinct muscular fibres, by which their separate move- 

 ments seem to be produced. At the base of the tentacular circle, just 

 above the anal orifice, is a small body (seen at A, a), which is a nervous 

 ganglion; as yet no branches have been distinctly seen to be connected 

 with it in this species; but its character is less doubtful in some other 

 Polyzoa. Besides the independent movements of the individual poly- 

 pides, other movements may be observed, which are performed by so 

 many of them simultaneously, as to indicate the existence of some con- 

 necting agency; and such connecting agency, it is affirmed by Dr. Fritz, 

 Miiller, 1 is furnished by what he terms a ( colonial-nervous system.' In 

 a Serialaria having a branching polyzoary that spreads itself on sea-weeds 

 over a space of three or four inches, he states that a nervous ganglion 

 may be distinguished at the origin of each branch, and another ganglion 

 at the origin of each polypide-bud; all these ganglia being connected 

 together, not merely by principal trunks, but also by plexuses of nerve- 



>See his Memoir in " Wiegmann's Archiv," 1860, p. 311; translated in 

 "Quart. Journ. of Microsc. Science," New Ser., Vol. i. (1861), p. 300. 



