ii8 naturalists' assistant. 



or one side of the skeleton may be broken in. Holothurians 

 should be first carefully examined, and the genital opening, 

 which is near the mouth, found. This genital opening should 

 be placed in the median line above, and then a longitudinal 

 incision made from the genital pore to near the anus. This 

 will expose the viscera and the parts will show a bilateral 

 symmetry. The relation of the longitudinal nerves and 

 canals, and the ambulacra should be studied by cutting 

 through the integument and one of the longitudinal muscles. 

 Clams are dissected by removing one valve. In order 

 that uniformity may be obtained the valve removed is the 

 left one. To ascertain the right and left, hold the clam with 

 the hinge from you, and the end from which the siphons ex- 

 tend at your right hand ; the upper valve will then be the left 

 one. Insert a dull knife in the gape of the shell and cut the 

 strong muscles which liold it closed. These will be found in 

 the clan near the hinge line, at the two ends of the shell. 

 In the mussel {Mytilus) and the oyster but one such muscle 

 will be found. The heart (near the hinge line) , the aliment- 

 ary canal with its tortuous course and the nervous system 

 may be then studied. The gills under the microscope show 

 a fine example of ciliary action. 



Snails should be extracted from the shell, by breaking it or 

 otherwise, and opened from the dorsal surface. 



The larger worms may nlso be opened from above, but 

 many of the smaller ones, especially among the lower forms, 

 must be studied in sections. 



Lobsters are opened by removing a portion of the carapax 

 exposing the circulatory apparatus; etc. The nervous system 



