164 



HOMOLOGY 



Head 



Antennae 

 Antennules 

 Mandibles or jaws 

 ist maxillae 

 2nd maxillae 



Thorax 



ist Maxillipedes 



2nd Maxillipedes 



3rd Maxillipedes 



ist Ambulatory (chelae) 



2nd Ambulatory 



3rd Ambulatory 



4th Ambulatory 



5th Ambulatory 



Copulatory ' 

 ist swimmerets 

 2nd swimmerets 

 3rd swimmerets 

 Abdomen \ 4th swimmerets 



5th swimmerets, or 



caudal appendages 

 Telson 



The terminal joint of the abdomen termed the telson, bears 

 the anus and is not usually regarded as a somite. 



Different as they are in function and different as they appear 

 to be in structure, the appendages are all built upon the same 

 plan, and throughout the series we can trace the same homolo- 

 gous parts. The simplest of all are the appendages of the 

 abdomen where three fundamental parts can be easily distin- 

 guished, a basal portion termed the protopodite attached to the 

 body, and two distal portions one of which is inside, that is near 

 the median line of the animal, the other outside. The internal 

 part is called the endopodite, the external part the exopodite 

 (Fig. 68). The first abdominal appendages show considerable 

 modification from the others in the male. Here the external 

 parts have disappeared leaving only the endopodites which are 

 tightly fused with the protopodites to form the copulatory organ. 

 In the female the appendages of this somite are degenerated as 

 shown by the entire absence of distal parts leaving only the 

 protopodites which are drawn out into plume-like organs. The 

 terminal appendages are similar to the other abdominal append- 

 ages but are much enlarged in all parts and strengthened by 

 j chitin and lime salts. 



The thoracic appendages are highly modified. All five of the 

 ambulatory consist of one distal branch only, the endopodite, 



