POLYZOA 907 



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 expan- 

 sion of a Flustra are devoid of contents and have lost their vitality, 

 whilst the edges are in a state of active growth. 1 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, or 

 they are developed from large portions of the inner surface of the 

 body-wall ; when mature they rupture, and set free the spermatozoa, 

 q q, which 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 or is 

 placed near the end of the c?ecal process of the stomach ; the ova, 

 having escaped from this into the visceral cavity, as at o, are fer- 

 tilised by the spermatozoa which they there meet with, and are 

 finally discharged by an outlet at/>, beneath the tentacular circle. 



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, i>, w, x, the direction and 

 points of attachment 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 sheath, when the animal has been completely withdrawn into its 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 con- 

 trary, appear to be chiefly accomplished 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 movements seem to be produced. At 

 the base of the tentacular circle, just above the anal orifice, is a small 

 body (seen at 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 polypides, other movements 

 may be observed, which are performed by so many of them simulta- 

 neously as to indicate the existence of some connecting agency : and 

 such connecting agency, it is affirmed by Dr. Fritz Miiller, 2 is fur- 

 nished by what he terms a ' colonial nervous system.' In a Set-'m- 

 laria having a branching polyzoary that spreads itself on seaweeds 

 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, 



1 For further details consult Haddon 'On Budding in Polyzoa,' Quart. Join >i. 

 Microsc. Sri. xxiii. p. ">!(>. Embryonic fission has been observed by Harmer in f'ri.tin 

 and Lichenopora. 



- See his memoir in Wici/nmnn's Archil', LstiO, p. 311, translated in Quart. Jonr/i. 

 nf Mirrnnr. Sci. n.s. vol. i. 1861, p. 300 ; Piev. T. Hincks's 'Note on the Movements of 

 the Vibracula in Caberea boryi, and on the supposed common Nervous System in 

 the Polyzoa,' Quart. Jtnirii. Microsc. Sri. xviii. p. 1. 



