50 A HALF-HOUR WITH THE 



This little plant is branched, and the stems present 

 a series of flattened cells. On the branches are 

 placed the fruit-bearing organs, in the form of 

 little capsules (a). These capsules contain the 

 tetraspores (6). At the ends of the branches are 

 organs of another kind, representing the stamens, 

 and which are called antheridia (e). The sea-weeds 

 present a great variety in the form of these organs, 

 and may be easily preserved for investigation in 

 small glasses of sea- water. 



The animal structures of the sea-water must 

 now, however, claim our attention. Amongst the 

 lowest forms of animal life are the sponges. They 

 are frequently cast on the shore with sea-weeds, 

 and afford interesting objects for the Microscope. 

 They are composed of animal matter, which lies 

 upon a structure of horny, calcareous, or sili- 

 ceous matter. The common sponge which is used 

 for domestic purposes may be taken as a type 

 of the whole group. If a thin section of the 

 common sponge is made with a pair of sharp 

 scissors and placed under a low power, it will 

 be seen to be composed of a network of horny 

 matter (Fig. 138, PL 5). If, now, we take any of 

 the common forms from our own sea-shore, we 

 shall find that the network is composed of sili- 

 ceous spicules lying one over the other (Fig. 139, 

 PL 5). If one of these spicules is examined (a) 

 and compared with a spicule from another sponge, 

 it will be found to differ in form and size ; and the 

 species of sponges can actually be made out by the 

 shape of their spicules. There is a little boring 

 sponge, called Cliona, found in the shells of old 

 oysters, which has its spicules pin-shaped (Fig. 140, 

 PL 5). The fresh-water sponge has very peculiar- 

 shaped spicula (Fig. 141, PL 5). In some, the 

 siliceous bodies are round, with projections, as in 



