122 ZOOLOGY 



SECT. 



together in such a way as to form a firm, elastic, supporting struc- 

 ture. They are secreted by the activity of certain cells in the 

 mesogloea which are called the sponginblasts, derived from the 

 ectoderm. In certain exceptional cases the spongin assumes the 

 form of spicules. The siliceous spicules (Fig. 92) are much more 

 varied in shape than the spicules of the Calcarea, and in a single 

 kind of Sponge there may be a number of widely differing forms of 

 spicules, each form having its special place in the skeleton of the 

 various parts of the Sponge-body. In most forms siliceous spicules 

 and spongin fibres combine to form the supporting framework, the 

 relative development of these two elements varying greatly in 

 different cases. But in certain groups, including the common 

 washing-sponges (Fig. 91, A), spicules are completely absent, and 

 the entire skeleton consists of spongin. In some forms which are 

 devoid of spicules, the place of these is taken by foreign bodies- 

 shells of Radiolaria, grains of sand, or spicules from other 

 sponges (Fig. 91, C). In others, again, such as the Venus's 

 Flower-Basket (Euplectetta), the Glass-Eope Sponge (Hyalonema), 

 and Pheronema (Fig. 93), the skeleton consists throughout of 

 siliceous spicules bound together by a siliceous cement. 



Reproduction in the Sponges is effected either sexually or 

 asexually. In some cases asexual multiplication takes place by 

 the production of external buds ; in others of internal buds in the 

 shape of groups of cells called gemmules, which eventually become 

 detached and develop into new individuals. In the Fresh-water 

 Sponges (Spongillidce) multiplication takes place very actively by 

 means of such gemmules, each of which is a spherical group of 

 cells enclosed in an envelope often with peculiarly-shaped siliceous 

 spicules, termed amphidiscs (Fig. 92, right side). These gemmules 

 are formed in the substance of the Sponge towards the end of the 

 year ; they are set free by the decay of the part of the parent 

 sponge in which they are developed, and fall to the bottom. In 

 spring the contained mass of protoplasmic matter reaches the 

 exterior through an aperture in the wall of the gemmule, and 

 develops into the adult form. 



All Sponges multiply by a sexual process by means of male 

 cells, or sperms, and female cells, or ova. 1 These are developed 

 from certain of the amoeboid wandering cells of the mesogloea, 

 which take up a special position, usually immediately below the 

 collared cells of the endoderm. Ova and sperms are developed in 

 the same Sponge, but rarely at the same time. The amoeboid cell 

 destined to form sperms divides into a number of small cells, giving 

 rise to a rounded mass of sperms. The latter, when mature, have 

 an oval or pear-shaped head and a long tapering appendage or tail. 

 Each amoeboid cell destined to form an ovum enlarges, and 



1 Sexually-produced larvae have not hitherto been found in the Hexactin- 

 elhda or Tetractmellkla. 



