240 PARASITIC HABITS OF CRUSTACEA. 
and most quadrupeds, birds and fishes used as food, are 
afflicted, and often suffer greatly from parasitic insects, crus- 
tacea and worms; and even man himself is likewise the prey 
of numerous parasites, both external and internal, some of 
which, like the Trichina spiralis, often cause painful dis- 
eases and even death. But the subject has also a pecüliar 
interest, when philosophically considered in connection with 
the varied phenomena of life and the theories of the origin 
of species. But at the present time it is our purpose merely 
to call attention to some curious facts concerning the habits 
of Crustacea, hoping that it may induce the readers of the 
Narurauisr to study more carefully the habits of this class, 
which, in this respect, is still very imperfectly known. 
It is certainly singular that a very great majority of all 
animal parasites belong to the Articulate division of the ani- 
mal kingdom, while very few are found among the Radiates, 
Mollusea, and Vertebrates. 
e three great classes of Articulates each have numerous 
parasitic representatives. The external parasites of land 
animals are mostly Insects, and their internal parasites are 
Worms; but the external parasites of aquatic animals are 
mostly Crustacea, while their internal parasites are both 
Crustacea and Worms. 
The class of Crustacea is naturally divided into three great 
groups, or subclasses. The highest, known as Decapods, 
have five pair of legs, hence their name, which signifies ten- 
footed. The lobsters and crabs are good examples. The 
next great group have seven pair ot legs, or are fourteen- 
wey hence their name Tefradecapods. The pill-bugs and 
sow-bugs are familiar land species. The lowest division, 
- : known as Entomostraca, have fewer mouth organs, and the 
. legs are irregular in number and position, while the abdomen 
has no appendages and often amounts to a mere spine, as in 
the Limulus, or “Horseshoe Crab,” which is a huge repre- 
: D essei of the gown dle most of the other species are 
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