76 A HALF-HOUR WITH THE 



the tentacula projecting like a many-parted tongue. 

 The polyzoa are also inhabitants of the fresh water. 

 Of these the most common form is the Plumatella 

 repens, figured at 163. The eggs of a fresh- water 

 species, Cristatella mucedo, seen in figure 164, are 

 covered with projecting spines with double hooks 

 at their extremities, perhaps for the purpose of 

 catching hold of objects. Such eggs may be often 

 found upon portions of water-lily, bulrush, and 

 other aquatic plants which float about in our rivers, 

 lakes, and ponds. 



Although but few of the shell-fish belonging to 

 the large class of mollusca are microscopic, yet the 

 structure of their shells can only be investigated 

 by the aid of the Microscope. 



If any common shell be picked up on the sea- 

 shore, it will be found to possess a rough outside, 

 generally of a darker colour, and sometimes beauti- 

 fully ornamented, whilst on the inside it is smooth, 

 and frequently of a rose-colour. This inner smooth 

 layer is called the nacre of the shell ; and it is 

 from this substance that pearls are formed in the 

 interior of many shells. Both the outer and the 

 inner layers present different kinds of structure in 

 different species of shells. The outer layer can be 

 well examined in the shell of the mollusc called the 

 Pinna. The outer layer in this shell projects be- 

 yond the inner, and may be easily submitted to 

 examination by reflected light under a low power, 

 when it will exhibit the appearance represented at 

 figure 166, plate 6. The external surface presents 

 the appearance of hexagonal cellular tissue. If a 

 portion of the shell is ground down, so as to form 

 a very thin layer, it may be examined with trans- 

 mitted light, and its hexagonal structure will be 

 much more apparent. If a portion be examined 

 lengthwise, it will be seen that the hexagons result 



