MICROSCOPE AT THE SEA-SIDE. 55 



cularia secure by their jaws the food necessary 

 for its sustenance. Of the various forms which 

 the cup itself assumes, none are more interest- 

 ing than those of the snake-head zoophyte (Fig. 

 157, PI. 6), in which it assumes the form of a 

 snake's head, with the tentacula projecting like 

 a many-parted tongue. The polyzoa are also 

 inhabitants of the fresh water. Of these the 

 most common form is Plumatella repens (Fig. 159, 

 PI. 6). The eggs of a fresh-water species, Cristatella 

 rnucedo, are covered with projecting spines with 

 double hooks at their extremities, perhaps for the 

 purpose of catching hold of objects (Fig. 160, PL 6).. 

 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 

 beyond the inner, and may be easily submitted 

 to examination by reflected light under a low 

 power (Fig. 161, PL 6). The external surface pre- 

 sents the appearance of hexagonal cellular tissue. 

 If a portion of the shell is ground down, so as to 



