ARTHROPODA 261 



Appendages of Apus cancriformis. 



1. 1st pair of antennae (antennules). 5. 2nd maxilla. 



2. 2nd „ „ 6-16. The 11 pairs of thoracic limts. 



3. Mandible. 17-68. The 62 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 

 Branohvpus 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 Eotifera, etc. 



Ordee 2. OSTRACODA. 



Chaeacteeistics. — Small, usually laterally compressed Entomo- 

 straca, with an unsegmented tody learing 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 : 



