ARTHROPOD A. 31 



eight thoracic and abdominal 'somites : ' and from the fact that 

 these areas cover in and protect the branchial chamber, they 

 have been called ' branchio-stegites/ Anteriorly the ' cephalo- 

 stegite' is produced into a long serrated rostrum. The sterna cor- 

 responding to the ' cephalo-thorax ' are, like the terga, confluent, 

 owing to the calcification of the membrane connecting the sterna 

 and epimera of adjacent somites. The sterna which are in rela- 

 tion to the three posterior pairs of ambulatory legs are tolerably 

 wide, but the sterna anterior to these become so narrow as to 

 allow the bases of the appendages to meet in the median line. 

 The calcified inter-epimeral, and inter-sternal membranes send 

 inwards calcified processes, which, after dividing, reunite with 

 each other in such a manner as to divide the floor of the 

 cephalo-thorax into a series of inter-communicating chambers. 

 A median canal is also formed by these processes, called the 

 ' sternal canal.' It contains the thoracic portion of the ventral 

 nervous cord, and is indicated by the insertion into it of a red 

 glass rod. 



The abdominal segments which have been already described 

 are tolerably simple, and only slightly modified : but those 

 appended to the 'cephalo-thorax' exhibit much greater diver- 

 sity of form, and subserve a variety of functions. 



Taking the segments in order, from behind forwards, and 

 beginning with the first sub-abdominal or fifteenth body- 

 segment, which has been already described, we find the fol- 

 lowing cephalo-thoracic appendages. 



XIV th pair. (Ambulatory legs.) 



Each appendage consists of a right ' protopodite ' and a long 

 segmented ' endopodite,' the ' exopodite ' being absent. The 

 joints of the appendage are named by Milne-Edwards as fol- 

 lows : 'coxopodite' ['protopodite'], which is attached to the 

 'somite,' and pierced by the external aperture of the 'vas 

 deferens,' into which a black arrow has been inserted : ' basi- 

 podite,' ' ischiopodite,' ' meropodite,' ' carpopodite,' 'propodite/ 

 and ' dactylopodite.' 



