GENERAL NOTES ON CRUSTACEANS. 313 
‘Decapods there are three pairs; a process of rapid growth 
and branching converts these into the compact digestive 
gland of the adult Decapods. In connection with the 
posterior end of the mid-gut in Amphipods and some 
others, there is a pair of blind tubes functioning as excretory 
organs, and presenting an interesting similarity to the 
Malpighian tubes of insects, which, however, are in con- 
nection with the hind-gut. The body cavity is never lage 
being mainly filled up with muscles and organs, and, as in 
In the blood, hemocyanin is the commonest pigment, ut 
is not universal. respiration is carried on in many 
different ways. In the simple forms it may be merely by 
the general surface, but in the majority of cases, certain 
portions of the limbs, or outgrowths of the limbs, constitute 
definite respiratory organs, often specialised to form gills.. 
In the excretory system the numerous nephridia of Annelids 
are absent. The typical excsesery—ergans of the Entomos- 
traca are the “‘shell glands”—paired coiled tubes opening 
on the second maxilla; of the Malacostraca, the antennary 
glands exemplified by the green glands of the crayfish. 
The genital ducts are possibly modified nephridia. 
There are many peculiarities connected with reproduc- 
tion—thus parthenogenesis for prolonged periods is common 
among “water-fleas” ; _herma hroditism is frequent, occur- 
often complicated_by the simultaneous existence o y 
often very diverse. The spermatozoa are often exceptional i in 
being very slightly motile. Some appendages are often 
modified for copulation or for carrying the eggs. 
Development.—The ova of most Crustacea show con- 
siderable similarity to those of Astacus, and the segmenta- 
tion is typically of the kind already described. But while 
this is the most typical case for Crustacean, and, indeed, 
for Arthropod development, it is possible, within the limits 
of the class Crustacea, to trace out a complete series, in 
which the first term is a segmentation of the complete 
and equal type, like that of a worm, and the last the 
purely peripheral. In the same way, though gastrulation 
is usually much disguised, there are many modes, from 
