GENERAL NOTES ON CRUSTACEANS. 3Ir 
characteristics, other than those facts of structure which 
we have already summarised. 
Admitting the parasitism of many Crustaceans, and the 
sedentary life of barnacles and acorn-shells, we must still 
allow that great activity characterises the class. With this 
may be connected the brilliant colouring, the power of 
colour change, and the phosphorescence of many forms. 
Except in the case of 
a few primitive and 
degenerate forms, the 
Crustacea are all seg- 
mented. In this, in 
the presence of hollow 
jointed appendages, in 
the reduction -of the 
coelom, and in their firm 
chitinous cuticle, the 
Crustacea resemble other 
Arthropeds ; as special 
characteristics we notice 
the two pairs of antennz, 
the presence of Carbdon- 
ate of lime in the cuticle, 
and the nature of the 
respiratory organs 
—these, with few excep- 
i i Fic, 165.—Nervous system of shore-crab 
ee sea usnrtes (Carcinus menas).—After Bethe. 
2 br., The supra-cesophageal mass; g., gullet 
While these characters surrounded. by gv, the gullet ring; mt. , the 
remain constant through- sub-cesophageal mass representing a fusion of 
S the thoracic ganglia of the crayfish, and 
out the group, there is giving off merc to the lint behind i it : 
1 j 7 a short strand representing the abdominal 
aly almost infinite variety ganglia of the crayfish. a1., antennules ; 
in detail. In regard a@?., antenna ; é., eye. 
to the segmentation of 
the body, we notice that, apart from the general tendency 
to reduction which is so marked in many parasitic forms, 
the higher forms as compared with the lower show marked 
specialisation. In the primitive Phyllopods the body con- 
sists of a large but varying number of segments, remarkably 
uniform in structure. The higher Crustacea, on the other 
hand, are characterised by their relatively few but constant 
