4 i2 THE MUSSEL CHAP. 



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

 frog, certain parasites. One of these, Polystomum (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, Ascaris (p. 153), 

 belongs to the phylum Nemathelminthes, in which the 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 

 Echinodermata? 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 exo- 

 skeleton 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 condition 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 

 40 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 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 except along the dorsal 

 hinge-line, where it forms an elastic ligavient connecting the two lateral 



