Introduction to A nimal Morphology. 329 



The nerve cells are unipolar, and the interior of the 

 ganglion consists mostly of granular substance : many 

 have a sympathetic system having its roots in the 

 ventral ganglia, and a separate vagus nerve at- 

 tached to the pharyngeal ring. The ova undergo 

 partial cleavage,* the undivided part of the yelk re- 

 maining within the elongated blastoderm, which early 

 shows a primitive streak and a division into symme- 

 trical lateral swellings. Except in Isopods and Am- 

 phipods the segments appear before their appendages. 

 The germ divides into two layers, the outer forming 

 the skin, muscles, and heart, the inner, the digestive 

 and reproductive (?) organs. Parthenogenesis occurs 

 in several cases, Metagenesis f in one. In most cases 

 the young undergo either progressive or retrogressive 

 metamorphosis. 



There are tw r o primary divisions of Arthropoda : 

 Division I. Branchiopnoa [Schmarda] water- 

 breathers,^: mostly with two pairs of jointed head 

 appendages or antennae. This includes the polymor- 

 phic class Crustacea (Latrcille\ characterised by hav- 

 ing the body divided into cephalo-thorax, abdomen, 

 and often post-abdomen. Each metamere usually 

 bears an appendage, even when it is fused with its 

 neighbours. The "crust" of each metamere consists 

 of a dorsal and a ventral lamina united at tin- ]><>im 

 of attachment of the limbs. In some the dorsal shell 

 of tWO ni'-<lian, /, /;;//, and t\v<> lateral, 







t H . Ci-cidorm 



Q-bearing. 



So: -iK-nts unite \vith 



the head, leaving the 1. 



