POST-LARVAL STAGES OF JASUS LALANDII. 121 



narrower, and shorter, and this has brought about a considerable change in 

 the relative positions o£ the gills. The rim of the thorax has disappeared as 

 such, and no longer forms a projection separating the pleurobranchs from 

 the arthrobranch. The position of the two pleurobranchs with reference to 

 each other has changed. The first or anterior, formerly only slightly further 

 from the arthrobranch than the second, is high up on the now vertical wall 

 of the thorax, while the second has remained low down. This rearrangement 

 of the pleurobranchs is, of course, in co-ordination with the shortening and 

 heightening of the side of the thorax (fig. 44). 



Fig. 44. 



ex. 



,.-pod. 



-arth. 



•ant.pL 



Positions of gills of Puerulus. 



With the disappearance of the rim of the thorax there is no clear separation 

 between the single arthrobranch and the nearest pleurobranch (the posterior), 

 so that it is now scarcely possible to draw any clear distinction between 

 arthrobranchs and pleurobranchs, and the functional joint has now become 

 enlarged to include the bases of these two gills. When calcification of the 

 sides of the thorax sets in, in later stages, the anterior gill is completely 

 separated from the other two. The adult condition may therefore be 

 described as brought about by the posterior pleurobranch of the phyllosoma 

 becoming an arthrobranch in the adult. It may, however, be noted here 

 that it is not quite correct to state that there are two arthrobranchs in Jasus, 

 as the posterior of these is marked off from the joint by a slight calcification 

 clearly seen in cast shells. 



In the next stage of Jasus lalandii the branchias have not altered much in 

 their position and relation to the coxa and its proximal joint. The cuticle 

 generally has become calcified, but the pleura only slightly. A slight deposit 

 of carbonate of lime is seen below the base of the first or anterior pleuro- 



