1894.] Paychology. 715° 
worm, which has all the powers of a most accomplished contortionist. 
The crayfish, after oviposition, carries its eggs beneath its tail, and the 
Histriobdella lives among them. Its office or function is to devour all 
blighted or unimpregnated eggs and dead embryos, the decay of which 
might affect the health of its host and progeny. Van Beneden, de- 
scribing the Histriobdella found associated with the lobster, says: 
“ Let us imagine a clown from the circus, his limbs dislocated as far as 
possible, we might even say entirely deprived of bone, displaying 
tricks of strength and activity on a heap of monster cannon balls, 
which he struggles to surmount; placing one foot formed like an air- 
bladder on one ball, the other foot on another, alternately balancing 
and extending his body, folding his limbs on each other, or bending 
his body upward like a caterpillar of the family geometride, and we 
shall then have but an imperfect idea of all the attitudes which it as- 
sumes, and which it Varies incessantly.” I once saw one of these little 
animals stand erect on its legs, then bend its body down between them 
and, with a quick flirt, turn a complete summersault. I have re- 
peatedly seen this mutualist insert its proboscis into the eggs of cray- 
fish and devour them. Microscopic investigation always showed that 
the eggs thus attacked were unimpregnated, consequently unfertile. I 
might prolong this paper by introducing many other mutualists, but 
think it hardly necessary. I have shown that these creatures subserve 
a very useful purpose in nature, and that they do not belong to that 
disreputable class—the parasites.— Jas. WEIR, Jux., M. D. 
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