220 



Arthropoda. Class 1. Crustacea. 



gills, and leaves it again at its anterior end ; the current is kept up 

 by the constant vibrations of the large flat setose exopod of the 

 second maxillae. The last (sixth) pair of abdominal legs, when 

 present, forms, with the terminal (seventh) segment, the broad 

 caudal fin ; of the other five pairs, the first and second of the male 

 are, as a rule, partially or completely modified to form copulatory 

 organs. The development of the abdomen is, in other respects, 

 very different in the different fonns. 



Pig. 182. PalcBunon. 1 — 7 abdominal segments, A^ — A^, first and second antennae, 

 A^ex exopod of the latter; H^, S^ third and sixth abdominal appendages; K^ third 

 thoracic appendage ( = third maxilliped) ; K^ fourth thoracic appendage ( = first ambulatory 

 limb); JETg eighth thoracic appendage (= 5th ambnlatoiy); iS carapace. — After H. Milne 

 Edwards. 



The Decapoda are furnished with a pair of auditory organs, 

 situated in the basal joint of the antennules. In many (Prawns, 

 Lobsters, and others), each is a depression of the skin which opens on tO' 

 the upper surface of the joint, and encloses peculiar jointed setee (auditory 

 hairs), which are set in motion by the sound waves. Resting upon 

 the auditory hairs are. grains of sand and the like, which are intro- 

 duced into the sac by the animal, and take the place of otoliths. In 

 others the vesicle is closed, but contains similar auditory hairs, and 

 sometimes an otolith secreted by its walls ; in yet other forms with 

 a closed sac (Crabs), the otolith is wanting. In the simplest cases of all 

 (certain Prawns) there is no depression at the spot corresponding with 

 the auditory sac of other forms, but there is a number of auditory 



