THE STRUCTURE AND LIFE-HISTORY OF SPONGES. 229 



according to Passelt, is decomposed when heated in air without 

 first becoming sticky. My own experiments show that it becomes 

 soft and sticky in superheated water (200°). 



The spongin always appears in layers. It surrounds in con- 

 centric layers, of different refracting powers, the spicule-bundles 

 of Halichondrida, as it does also the axial thread or pith cylinder 

 of Horny Sponges. It is secreted by gland-cells. These elements 

 are pear-shaped, and similar to the gland-cells of the skin ; the 

 protoplasm is dense and granular, the nucleus large and spherical. 

 The cells are attached by a slender peduncle to the surface of 

 the fibre, they are pretty closely packed, and form a more or less 

 continuous mantle investingthe growing parts of the skeleton fibres. 

 These cells have been termed " spongoblasts" by their discoverer, 

 F. E. Schulze. They occur only on those parts of the fibrous 

 skeleton which are still growing, and disappear as soon as the 

 fibres attain their full size. The solid reticulate skeleton 

 of the Sponcjidis, known to everyone as the Bath Sponge, con- 

 sists of a few thick, radial, so-called main fibres, between which 

 a fine network of connecting fibres is spread out. In all the 

 fibres we can distinguish an axial thread which consists of a 

 granular substance, and which is surrounded by spongin. At the 

 joining points of the fibres we see that the layers are not continuous, 

 and that all the axial threads are not in connection with each 

 other. The main fibres grow principally at the ends in length, 

 and afterwards in thickness ; the connecting fibres rapidly reach 

 their full thickness, and do not grow in length at all. The 

 axial threads, on the surface of which the spongin is precipitated, 

 form a network, but they are in no connection with the axial 

 threads of the main fibres. In the latter, foreign bodies are 

 often found which are used by the sponge as material to build 

 up its own skeleton, and which are cemented together with 

 spongin. Tbey are selected from the bodies which accidentally 

 fall on the surface of the sponge, chiefly sand-grains, Foraminifera- 

 shells, and siliceous spicules of other sponges. 



Spermatozoa and ova are observed in sponges. The sper- 

 matozoa possess rounded or sharp-pointed, slender heads. They 

 are formed by the continued fission of spherical mesoderm cells, 

 derived from amoeboid wandering cells. In the Calcareous 

 Sponges these cells divide first into two— a sperm mother-cell 

 and a covering ceU. By continued division the spermatozoa 



