804 THE ANIMAL KINGDOM. 



sponge. (2) In Tetilla radiata, T. euplocamus (Ancorinidae), and Tethya 

 maza (Tethyadae) mesoglaeal cells multiply and form a mass near the 

 surface of the sponge. They surround in the Tetillae in question 12-20 

 ampullae, but in the Tethya ampullae grow out among the cells and then 

 multiply in number. The bud-mass is gradually extruded either along a 

 fascicle of spicules belonging to the parent, or by an outward movement of 

 the fascicle. It has an ectoderm ; spicules develope in it and in the 

 Tethya named subcortical spaces. In Tetilla japonica the mass of cells 

 contains no ampullae, nor does it in Tethya lyncurium, where it originates 

 by the division of a single cell. Buds of a similar external character, the 

 histology of which is not known, occur also in two or three other species 

 of Tetkya,'m a Suberites, and in Rinalda {Polymastia ?) arctica. They 

 are developed in the last-named at the apex of hollow conular processes of 

 the sponge, the walls of which are perforated by pores x ; and there are 

 usually two or three buds in a line, one beyond the other, connected by 

 spicules, and a small quantity of sponge-substance, a phenomenon seen 

 also occasionally in a Tethya from the White Sea. In Craniella Millleri 

 (= Tethya s. Tetilla cranium) buds of the same kind develope into sponges 

 within the parent. The freshwater Spongillidae, with one or two exceptions, 

 develope resting buds, the seeds or gemmules, at the commencement of 

 the winter in cold regions, of the hot season in tropical. They possess a 

 protective envelope or case, an organic membrane hardened, at least in 

 some instances, by silica, generally strengthened by siliceous spicules as 

 well, often complex in structure and provided with a single aperture, or 

 with a principal and several secondary apertures. A hydrostatic apparatus, 

 or vesicular dilatation of the special membrane or cuticle, which im- 

 mediately invests the contents of the case, closes the aperture in two 

 instances. The envelope is said to be formed by the outer layer of cells of 

 the gemmule in Meyenia (Spongilla) fluviatilis, by mesoglaeal cells in 

 Spongilla lacustris. Its contents are granular cells, closely packed, derived 

 from mesoglaeal and transformed endodermal cells. When climatic con- 

 ditions are favourable, they escape and reproduce the sponge. See p. 25 o 2 . 

 The production of gemmules brings about the death of the sponge. 



A peculiar mode of propagation has been observed in Oscarella 



1 Merejkowsky thinks that these conular processes are converted into oscular tubes, after having 

 served as supports for the buds : Mem. Imp. Acad. St. Petersburg, (7), xxvi. No. 7, p. 10. 



2 Goette, Wierzejski, and Marshall do not quite agree in their accounts of the mode in which 

 the gemmule and its case are formed. See the summary by Vosmaer, Porifera, Bronn's Thierreich, 

 ii. pp. 428-9 ; Goette, Abhandl. Entwickelungsgeschichte der Thiere, Leipzig, pt. 3, 1886, p. 21 ; 

 Wierzejski, Archives Slaves de Biologic, i. 1886; Marshall, SB. Naturf. Ges. Leipzig, 1884, 

 pp. 22-9, or Journ. Royal Micr. Soc. (2), v. 1885, p. ion. 



Gemmules were said by O. Schmidt to occur in a marine Reniera (Z. W. Z. xxv. Suppl. p. 139) : 

 but Keller states that he observed similar structures in a dead Chalinulafertilis, and that they proved 

 to be the egg-masses of a Dinophilus (Z. W. Z. xxxiii. p. 341). 



