STRUCTURE OF POLYZOA '. — LAGUNCULA. 579 



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 produc- 

 tion of Gemmae from the Polypides themselves, the best examples 

 are furnished by the Flustrce and their allies. From a single cell 

 of the Flustra, five such buds may be sent-off, which develope 

 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 a very large amount. 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 contents 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 Genera- 

 tion ; the sexes, however, being usually, if not invariably, united 

 in the same Polypides. The Sperm-cells are developed in a glan- 

 dular body, the Testis m, which lies beneath the base of the 

 stomach ; when mature, they rupture, and set free the Sperma- 

 tozoa 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 ; the ova, having escaped from this into the visceral cavity, 

 as at o, are fertilized by the spermatozoa which they there meet 

 with ; and are finally discharged by an outlet at p, beneath the 

 tentacular circle. 



448. 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 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 withdrawn into its interior.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 contrary, 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 Tentacula 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, a), 

 which is a Nervous Ganglion ; as yet no branches have been dis- 

 tinctly 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 simultaneously 

 as to indicate the existence of some connecting agency ; and such 

 connecting agency has lately been detected by Dr. Fritz Midler, * 



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

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



p p 2 



