ARTHROPODA 261 
Appendages of Apus cancriformis. 
1. 1st pair of antennae (antennules). 5. 2nd maxilla. 
7, Pratl a ‘A 6-16. The 11 pairs of thoracic limbs. 
3. Mandible. 17-68. The 52 pairs of abdominal limbs. 
4. 1st maxilla. 
The annulations of the abdomen are much fewer than the 
number of appendages; the nerve ganglia, however, correspond 
with the limbs. The last two or three segments carry no 
appendages. The two nerve cords are separated by a consider- 
able interval, being connected by transverse commissures at the 
ganglia. The heart is elongated, and extends through the 
eleven thoracic segments, with a pair of ostia in each. In 
Branchipus the heart extends throughout the whole body. 
The Branchiopoda are frequently parthenogenetic, and the 
males are much rarer than the females; one species of Apus 
has recently been shown to be hermaphrodite, the posterior end 
of the generative gland producing spermatozoa. They inhabit 
freshwater lakes and pools, and one genus, Artemia, lives 
in brine pools, in which the salt may be so concentrated as to 
be fatal to all other forms of life. Their eggs are capable of 
surviving long periods of drought, embedded in the dried-up 
mud: a property they share with those of the Cladocera, 
the Cyclopidae, and the Rotifera, ete. 
OrpER 2. OSTRACODA. 
CHARACTERISTICS.—Small, usually laterally compressed Entomo- 
straca, with an unsegmented body bearing seven pairs of 
appendages ; the abdomen is rudimentary. The whole body 
is enclosed in a bivalved shell. 
The group Ostracoda includes a great number of genera 
and species, but it is nevertheless a very homogeneous assembly, 
the various species differing but little from one another. The 
bivalved shell which encloses the animal has a striking re- 
semblance to that of some Lamellibranchs; the valves are 
divaricated by means of an elastic ligament which occupies 
about the middle of the dorsal surface, and are occluded by an 
adductor muscle. 
The body is not segmented, but head, thorax, and a rudi- 
mentary abdomen can be distinguished. The appendages are 
as follows : 
