CHAR A CTERISTICS. 53 1 



cells with slender branching processes, which unite with those of other cells to 

 form a network ; wandering amoeboid cells, probably concerned with digestion, 

 distribution of nutriment, and excretion; skeleton-forming cells, which secrete 

 lime, or flint spicules, or horny fibres ; contractile muscle-cells, possibly nerve- 

 cells ; and, lastly, male and female reproductive cells. The cells with the slender 

 processes secrete the gelatinous ground-substance, which may be compared to the 

 material forming the umbrella of jelly-fish. 



The varying consistency of sponges, which may be soft, stony, leathery, horny, 

 etc., results from the amount and kind of material secreted by cells of the mesoderm. 

 In the Venus' flower-basket, these cell architects form large spicules of silica which 

 are joined into a trellis. In the living bath-sponge, groups of cells congregate in the 

 ground-substance, and secrete a network of cylindrical fibres of horny material. In 

 calcareous sponges, the skeleton-cells form spicules which nearly always remain 

 separate, and are always beautifully adapted for purposes of support. In addition to 

 forming a support, the skeleton spicules, in many cases, afford a means of defence 

 against small animals by forming spikes in the canals, or on the surface ; and, further, 

 it is improbable that any fish would repeat the experiment of eating a siliceous sponge. 



To return to the bread-crumb sponge. A dried specimen can easily be 

 crumbled into powder, in which can be seen numerous glassy spicules, pointed at 

 each end, and about an eightieth of an inch in length. The spicules, which are 

 unaffected by most of the strong acids, are composed of silica, and are allied in 

 composition to flint and opal. 



Each needle is made up of concentric laminae of silica, deposited round a fine 



central axial canal containing a thread of organic matter, and each is formed in 



a cell of the mesoderm. In this sponge the needles are separate, and scattered 



with scarcely any regularity in the ground-substance, excepting at the surface, 



where bundles of needles are joined by their ends to form the gauze-like network. 



In many sponges the rods or bundles of rods form a regular scaffolding. 



As regards the modes of reproduction, both male and female 

 Reproduction. . * 



cells are found in the mesoderm, either in the same or in different 



specimens. The male cells in sponges generally give rise, by division of the 



nucleus, to masses of spermatozoa, each of the latter possessing a conical head 



and a long vibratile tail. The ova appear as large rounded cells, which, after 



fertilisation, undergo segmentation or division, first into two cells, and each of 



these again into two, and so on, until a mass of cells results, two kinds being 



present, one forming an outer layer covering the other. The outer layer of the 



now egg-shaped embryo, excepting at the narrow end, is composed of long, 



narrow, cylindrical cells, provided with cilia; and the inner mass is composed 



of large granular cells. The embryos appear as minute oval bodies, about the 



size of a pin's head. If a bread - crumb sponge be cut open in the autumn, 



they will be seen as bright yellow spots in the body-substance. By keeping 



specimens in a vessel of water, and examining them daily, the embryos will 



be observed being driven out of the oscules, and swimming about with the broad 



end forwards. After from twenty - four to forty - eight hours of independent 



roving existence, an embryo fixes itself by its broad end, and becomes flattened. 



By a remarkable transformation peculiar to sponges, the large granular cells of the 



