2S CRUSTACEA. 



examination, looks as if it might be made of the lower 

 portions of several rings which have become soldered 

 together. A number of these rings may be counted, 

 if each appendage is carefully examined at the point 

 of its attachment, and afterwards removed. 



The great difficulty, at this stage of the investigation, 

 is to lead the scholars to suspect that these internal 

 rings do not belong to the shield above. That this 

 difficulty may be overcome, let them examine the 

 structure of the carapace. bet them see for them- 

 selves that it is a shield formed of the skin folded, and 

 hanging down on either side, so as to cover the cavi- 

 ties in which lie the gills or breathing organs. This 

 can be done by careful use of the knife, or scissors, in 

 cutting away the pendant sides of the shell, or by using 

 lobsters picked out for the purpose, which have the 

 thinnest and most pliable shells, which can be lifted 

 and bent without tearing or breaking; also by placing 

 a fresh or alcoholic specimen in dilute acetic acid until 

 the chalky parts of the shell have been dissolved. In 

 the latter condition it is easy to lift up the side flaps of 

 the shield, and show that they are merely lateral folds 

 of skin. 



Another point is to lead them to observe how all the 

 depressions or sutures in the abdomen run straight 

 across the body to the appendages, and how all in the 

 shield run forward towards the mouth. This is shown 

 most plainly by the central, transverse suture (big. 6, 

 su) which the pupils observed in the carapace at the 

 beginning of the lesson. This suture marks the limits 

 of a ring, and its direction indicates that the ring 

 belongs to one of the pairs of appendages in the 



