1899-1900.] Report of the Microscopical Section. 123 
antenne, or feelers, which act as organs of touch. The next 
somite in order bears a pair of mandibles, or jaws, which are 
actuated by a pair of very powerful muscles. These are the 
principal external organs for crushing the food of the crayfish. 
The fifth and sixth somites bear each a pair of maxille, or 
lesser jaws. 
The next somites in order are situate in the region of the 
thorax, and of these the seventh, eighth, and ninth bear each a 
pair of maxillipedes, or foot jaws, so called from being organs 
of mastication and at the same time having a general resem- 
blance to legs. They are called the first, second, and third 
maxillipedes, of which the last is strongly toothed along the 
inner edge. The tenth somite bears the great forceps, or 
chele, the principal prehensile organs of the crayfish. The 
chelate part of these organs, or hand as it is sometimes termed, 
is moved by very powerful muscles called the adductor and 
abductor muscles, the former of which especially is attached 
to a very powerful tendon, thus enabling the animal to seize 
_ and hold its prey very securely. The next four somites bear 
each a pair of ambulatory or walking legs, and of these the 
_ first two pairs are chelate, and thus help in tearing the food ; 
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out 
1 
_ the other two pairs end in simple claws. 
With the fifteenth somite begins the first of the abdominal 
ones, of which there are six. To each of these, except the 
last, are attached a pair of small appendages called swim- 
merets. The last somite has two large broad plates attached, 
one on each side of the animal, with a third plate between 
called the telson, and these three plates constitute the flapper 
of the crayfish, by aid of which it is enabled to move very 
‘Yapidly backwards. By some naturalists the telson, with the 
plate on each side, is held to constitute an additional somite, 
and those who so hold reckon the number of somites as 
twenty-one. 
The exoskeleton, or outer covering of the crayfish, is a 
calcified hard substance, except at the joints, where it is soft 
and flexible. If a bit of the calcified part is put into a weak 
solution of chromic acid for a week or ten days, it becomes 
soft, and thus thin sections of it can be made for examination 
under the microscope. Under a high power, say of one-eighth 
or one-tenth of an inch, cross sections exhibit a beautiful 
Sat 
