168 



HOMOLOGY 



r Antennae 

 I Antennules 

 Head { Mandibles or jaws 



ist maxillae 

 I 2nd maxillae 



Thorax 



Abdomen 



ist Maxillipedes 



2nd Maxillipedes 



3rd Maxillipedes 



ist Ambulatory (chelae) 



2nd Ambulatory 



3rd Ambulatory 



4th Ambulatory 



5th Ambulatory 



Copulatory 



ist swimmerets 



2nd swimmerets 



3rd swimmerets 



4th swimmerets 



5th swimmerets, or 

 caudal appendages 



Telson (unpaired) 



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 homologous 

 parts. The simplest of all are the appendages of the abdomen, 

 where three fundamental parts can be easily distinguished, 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) . 

 In the male, the first abdominal appendages show considerable 

 modification from the others. 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 

 chitin and lime salts. 



The thoracic appendages are highly modified. All ten of the 

 ambulatory consist of one distal branch only, the endopodite, 



