98 



ESSENTIALS OF ZOOLOGY 



the first or penial pair. In the female, the second pair have the 

 same structure as the next three. The third to the fifth have 

 all a similar structure in both sexes, but they are larger in the 

 female than in the male. They each consist of a two-jointed 

 protopodite bearing two flattened oval setose plates, the endo- 

 podite and the exopodite. The sixth abdominal appendages, 

 the uropoda, are highly developed, but they consist nevertheless 

 of a strong broad protopodite, produced dorsally into an outer 

 and an inner spine, and of the two large plates on each side 

 of the telson. These are the endopodite and the exopodite, 

 and the latter is subdivided near its extremity by a transverse 

 joint. The joint is further distinguished by the row of small 

 spines on the distal edge of the large basal portion. 



The Gills. The gills are borne by the appendages of the 

 thorax and the segments corresponding to them. They are 

 outgrowths of the integument, and each consists of a slender 

 column, containing the two blood-vessels and bearing a dense 

 mass of branchial filaments arranged for the most part in two 

 rows. They project upwards, side by side, in the branchial 

 chamber (formed by the downgrowth of the carapace the 

 branchiostegite), each segmental group being separated from 

 the next by the epipodite. They are arranged in three layers : 

 (1) the outer series, the podobranchs, borne by the proximal 

 segment (coxopodite) of the appendage in each case; (2) a 

 middle series, the arthrobranchs, arising from the thin integu- 

 ment forming in each case the joint between the limb and the 

 body ; and (3) an inner series, the pleurobranchs, attached to 

 the body wall. 



The gill formula of the Norway lobster is : 



Each podobranch arises from the base of the laminar 



