120 



DR. J. D. F. GILCHRIST ON LARVAL AND 



The shape o£ the epimeron is intermediate between that o£ the Inst 

 phyllosoma and the adult (fig. 42). 



The gills of the pnerulus ai-e of: interest, as they seem to point to the 

 solution of a difficulty connected with the difference in position of the gills 

 in the phyllosoma and the adult. In the adults of the Scyllaridse and all 

 decapods generally there are (it is believed) one pleurobranch, two arthro- 

 branchs, and one podobranch, while in the larval forms (phyllosomas) there 

 are clearly two pleurobranchs, one arthrobranch, and one podobranch. This 

 difference between the young and adult forms is well known, and it is 

 desirable to ascertain whether the adult condition is a primary or secondary 

 one. There can be no doubt that the arthrobranch of the phyllosoma is 

 placed on the joint (fig. 43) and that it is well separated from the two 

 pleurobranchs. These latter occur on the dorsal side of the flattened thorax, 

 and the name is therefore not strictly accurate. The real side or pleuron of 



pod.^ /B 



post. pL- 



Gills of Phyllosoma of 35 mm. 



the thorax is still in the form of a thick cuticular rim, which, although 

 rather thinner at the insertion of the appendage, still forms a marked ridge 

 separating the arthrobranch clearly from the pleurobranchs, so that frequently, 

 when viewed from above, only part of the arthrobranch can be seen, pro- 

 truding from under the edge of the thorax, which overhangs the base of the 

 limb. The two pleurobranchs (on the 2nd to the 4th walking-legs) are at 

 some distance from this edge and are thus clearly separated from the joint 

 and its gill-rudiment. They are also well separated from each other, and it 

 is particularly to be noted that the posterior is distinctly nearer the side 

 of the body than the anterior. This has been noted in all the phyllosomas 

 examined and may occur in other phyllosomas, though the point has not been 

 particularly noted. 



In the puerulus stage the thoracic region has become much deeper. 



