412 THE MUSSEL CHAP. 



In addition to these, you will probably have seen, when 

 dissecting the frog, certain parasites. One of these, Poly- 

 stomum (p. 33), belongs to a lowly group of worms of a 

 flattened form, constituting the phylum Platyhelminthes, 

 which includes the parasitic liver-flukes and tape-worms 

 as well as certain free-living forms ; another, Rhabdonema 

 (p. 153), belongs to the phylum Nemathelminthes, in which 

 the mostly parasitic thread-worms are placed. Apart 

 from certain other smaller groups, which include such 

 animals as " wheel-animalcules," " sea-mats," " lamp- 

 shells," &c., there only remains one other of the larger 

 phyla of which you will not have studied an example, viz., 

 the phylum Eehinodermata, which is constituted by the 

 star-fishes, sea-urchins, sea-cucumbers, &c. : these are all 

 inhabitants of the sea, and are peculiar in exhibiting in the 

 adult a more or less pronounced radial arrangement of 

 their parts (p. 296), and in possessing a curious calcareous 

 exoskeleton developed within their integument, consisting 

 of small particles or of definitely-shaped plates. All the 

 phyla with the exclusion of the Vertebrata are spoken of 

 collectively as the Invertebrata. 



PRACTICAL DIRECTIONS 



FRESH-WATER MUSSEL 



Examine a living specimen in a vessel of water with some 

 sand at the bottom, and note the greenish-brown bivalve 

 shell, the foot, and the manner in which the animal buries 

 itself, anterior end downwards, with the pallial openings 

 projecting posteriorly. Observe the currents of water 

 passing in at the fringed inhalant aperture, and out at the 

 exhalant aperture : these can be more easily observed if a 

 little finely-powdered indigo or carmine is placed in the 

 water. 



In order to kill the animal in as fully-extended a condi- 

 tion as possible, place it in a saucepan or beaker of water, 

 and heat over a gas-burner or spirit-lamp until the water is 

 warmed up to a temperature of about 4O 3 C., when the foot 

 will be protruded. Keep at this temperature until the 

 mussel is dead. The animal is best preserved from day to 

 day in 70 per cent, spirit, or 2-3 per cent, formaline. 



A. External Characters. 



Compare the shell with the carapace of the crayfish, and 

 observe that it covers the whole body and is densely calcified 



