220 ANNULATA. 



There are nineteen pairs of gills. They are situated 

 along the sides of the thorax and are protected by the 

 Respiratory t>ranchial plate of the carapace. The branchial 

 chamber so formed communicates freely with 

 the exterior between the legs and at the hind end, but the 

 principal aperture (the cervical canal) lies at the front end 

 and opens beside the mouth. 



In this there lies the scaphogfiathite of the 2nd maxilla 

 which is said to bale or scoop the water out of the branchial 

 chamber, fresh water coming in from behind and between 

 the legs. Each gill consists of a central axis with lateral 

 branches, covered with a very thin cuticle, ectoderm and 

 mesodermic layer. In its interior the blood circulates from 

 afferent to efferent branchials. 



Five of the gills are fixed to the bases of the third 

 maxillipede, chela and first three legs. They are termed 

 podobranchs. To the arthrodial membrane of each of 

 the same appendages is attached a pair of small arthro- 

 branchs \ whilst higher up, on the side-wall of the thorax, 

 are found four large pleurobranchs^ which are supposed 

 to correspond to the four last segments.'^ The epipodites lie 

 between each set of gills in each segment and force the in- 

 coming water to pass the whole way up the gills instead of 

 taking a short cut to the cervical canal. 



It is possible that the primitive arrangement was that 

 of a podobranch, two arthrobranchs and a pleurobranch to 

 each segment of the thorax, making a total of thirty-two, 

 but this number persists only in numbers 5, 6 and 

 7, where the thorax is broadest. The cavity has 

 become narrower in front and behind, hence the last leg 

 loses its podobranch and its two arthrobranchs, and the 

 pleurobranchs all disappear in front of number 5 : so also 

 do the arthrobranchs and podobranchs in the first two seg- 

 ments. With a loss of ten gills in front and three behind, 

 the thirty-tw^o is reduced to nineteen. This will be clear 

 after an inspection of the diagram. t 



* It is probable that all the gills arise on the basal joint of the thoracic appen- 

 dages, but the pleurobranchs and arthrobranchs migrate during development to their 

 final positions. 



t In Astactis there is a podobranch on the second maxillipede and one arthro- 

 branch above it, but only the last pleurobranch remains ; thus it possesses only i8 

 pairs of gills, i.e., podobranchs (6), arthrobranchs (ii) and pleurobranchs (i). 



