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 a. 

 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 Grantia 

 ciliata. There is a little boring sponge, called 



