STRUCTURE OF LAGUNCULA. 



493 



animals of the Polyzoa, in consequence of their universal ten- 

 dency to multiplication 



by gemmation, are sel- FIG. 246. 



dom or never found soli- 

 tary, but form clusters 

 or colonies of various 

 kinds ; and as each is en- 

 closed in either a horny 

 or calcareous sheath, or 

 " cell, " a composite 

 structure is formed, 

 closely corresponding 

 with the polypidom of 

 a Zoophyte, which has 

 been appropriately de- 

 signated the " polyzo- 

 ary." The individual 

 cells of the "polyzoary" 

 are sometimes only con- 

 nected with each other 

 by their common relation 

 to a creeping stem or 

 " stolon," as in Lagun- 

 cula (Fig. 246) ; but more 

 frequently they bud forth 

 directly, one from an- 

 other, and extend them- 

 selves in different direc- 

 tions over plane surfaces, 

 as is the case with Flus- 

 trce, Lepralice, &c. (Fig. 

 245) ; whilst not unfre- 

 quently the Polyzoary de- 

 velopes itself into an ar- 

 borescent structure (Fig. 

 247) which may even pre- 

 sent somewhat of the den- 

 sity and massiveness of 

 the stony Corals. Each 

 individual is composed 

 externally of a sort of 

 sac, of which the outer 

 or tegumentary layer is 

 either simply membran- 

 ous, or is horny, or in 

 some instances calcified, 

 so as to form the cell ; 

 this investing sac is lined 

 by a more delicate mem- 



W 



Laguncula repens, as seen in its expanded slate at A, 

 and in its contracted state, in two different aspects, at B 

 and c. The same references answer for each figure : 

 a a, tentacula clothed with vibratile cilia; b, pharyngeal 

 cavity ; c, valve separating this cavity from d the oesopha- 

 gus ; e, the stomach, with / its pyloric valve, and g the 

 circle of cilia surrounding that orifice; h, wall of the 

 stomach with biliary follicles; i, the intestine, contain- 

 ing k excrementitious matter, and terminating at I the 

 anus; m, the testicle ; n, the ovary; o, an ovum set free 

 from the ovary; _p, openings for the escape of the ova; 

 q, spermatozoa freely moving in the cavity that surrounds 

 the viscera; r, retractor muscle of the angle of the aper- 

 ture of the sheath ; s, retractor of the sheath; t, retractor 

 of the tentacular circle; M, retractor of the oesophagus; 

 v, retractor of the stomach ; w, principal extensor 

 muscle ; x, transverse wrinkles of the sheath ; y, fibres 

 of the sheath, themselves probably muscular; z, 

 muscles of the tentacula; a (at the base of the tentacular 

 circle in A), nervous or cesophageal ganglion; ft, stem, 

 D, a portion of the tentacular circle shown sepa- 

 rately on a larger scale; a a, the tentacula clothed with 

 cilia: b b, their internal canals ; c, muscles of the tenta- 

 cula; d, transverse muscles forming a ring at the base 

 of the tentacula: e, muscles of the tentacular circle. 



