176 THE MORPHOLOGY OF THE COMMON CRAYFISII. 
of the body consists of one or more of the following eight 
histological groups :— 
1. Blood corpuscles; 2. Epithelium; 3. Connective 
tissue; 4. Muscle; 5. Nerve; 6. Ova; 7. Spermatozoa; 
8. Cuticle. 
1. A drop of freshly-drawn blood of the crayfish con- 
tains multitudes of emall particles, the blood corpuscles, 
2) Bed il 
Fic. 49.—Astacus fluriatilis.—The corpuscles of the blood, highly 
magnified. 1—8, show the changes undergone by a single cor- 
puscle during a quarter of an hour; , the nucleus; 9 and 10 
are corpuscles killed by magenta, and having the nucleus deeply 
stained by the colouring matter. 
which rarely exceed 1-700th, and usually are about 
1-1000th, of an inch in diameter (fig. 49). They 
are sometimes pale and delicate, but generally more or 
less dark, from containing a number of minute strongly 
refracting granules, and they are ordinarily exceedingly 
irregular in form. If one of them is watched continu- 
