MICROSCOPE AT THE SEA-SIDE. 53 



are situated, are called polypidoms. It is the 

 polypidom which constitutes the coral in the 

 family of polyps, producing the masses of carbonate 

 of lime which sometimes cover the bottom of the 

 ocean and form reefs in the sea. In one family of 

 polyps, known as sea-fans (Gorgonice), the cal- 

 careous polypidoms contain spicula of various forms, 

 which are beautiful objects under the Microscope 

 (Fig. 148, PL 5). 



Another family of animals common enough in 

 the sea, are the star- fishes and sea-eggs. Although 

 not themselves microscopic, certain parts of their 

 structure present very interesting objects for ex- 

 amination. If a section is made of one of the 

 spines of the common echinus, or sea-egg, it pre- 

 sents under a low power a beautifully radiated 

 structure (Fig. 149, PL 5). The suckers, also, of the 

 same animal present little rosett.es (Fig. 150, PL 5). 

 Upon the surfaces of both star-fishes and sea-eggs 

 will be found little movable bodies which are 

 called pedicellarice. In the sea-egg they possess 

 three movable nipper-like limbs (Fig. 151, PL 5), 

 whilst in the common star- fish they present only 

 two (Fig. 152, PL 5). A controversy has been 

 raised on the question as to whether these bodies 

 are parasitic animals, or part and parcel of the 

 structure of the creature on which they are found. 

 As they are so constantly present, they acre pro- 

 bably parts of the animal on which they are found. 

 The movements of these nippers are very active, 

 and they frequently lay hold of objects which pass 

 near them. 



As common on the shore as the potypidoms of 

 the polyps, are the animal skeletons called sea- 

 mats. When placed under a low power, and 

 viewed by reflected light, the sea-mat is com- 

 posed of little cavities or cells (Fig. 158, PL 6). 



