3506 THE SPONGES 



mesoderm cells, and not to constitute a special separate layer distinct in itself. 

 Whether this be the case or not, the terms mesoderm and ectoderm may be conven- 

 iently retained here. The ground substance, or mesoderm, contains cells of various 

 kinds, namely, irregularly-shaped cells with slender branching processes, which 

 unite with those of other cells to form a network; wandering amoeboid cells, proba- 

 bly 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 jellyfish. 



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 meso- 

 derm. In the Venus' s 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 defense against small animals by forming spikes in the canals, or 

 on the surface; and, further, it is improbable that any fish would repeat the experi- 

 ment 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 un- 

 affected by most of the strong acids, are composed of silica, and are allied in com- 

 position 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 cells 

 are found in the mesoderm, either in the same or in different speci- 

 mens. 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 fertilization, 

 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, pro- 

 vided with cilia; and the inner mass is composed of large granular cells. The em- 

 bryos 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; 



