70 A HALF-HOUR WITH THE 



which is represented at figure 111, plate 4. As 

 seen in the drawing, 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, seen at . 

 These capsules contain tetraspores (d). At the 

 ends of the branches are organs of another kind, 

 representing the stamens, and which are called 

 antheridia. These are seen at e in the same figure. 

 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 form 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, repre- 

 sented in figure 140, plate 5. If now we take one 

 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, as repre- 

 sented in figure 141 of plate 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. Some of our British sponges have cal- 

 careous spicules. This is the case with Graitfia 

 ciliated There is a little boring sponge, called 



