POST-PUERULUS STAGE OF JASUS LALANDII. 197 



the hinge of the mandible. The three prominences or teeth of the mandible 

 are more marked, and a cutting-edge on -which there is a series of small 

 denticles of irregular shape, lias appeared between the second and third. A 

 few of these appear also on the first and second tooth in some specimens, not 

 in all, and, at their base, a large one was in all cases found, ending in three 

 sharp points. These denticles are readily distinguished from the rest of the 

 thickened cuticle, on which they lie, as they are of a dark brown colour. 

 Doubtless their appearance is associated with the change in the feeding 

 habits of the animal. They disappear in the subsequent stages, in which the 

 mandible has become calcified. 



The change in the third tooth is marked. From a mere undulation of the 

 cuticle in the puerulus it has now become prominent and conical, with a 

 cleft apex, and is situated behind the cutting denticulate outer margin of 

 the mandible. The part which it takes in the formation of the adult mandible 

 is now apparent, for in the latter the main part of the molar surface is formed 

 by a cleft or V-shaped prominence, which is apparently the transformed 

 third tooth. The inner limb or ridge of the prominence forms the thick 

 posterior margin of the molar surface. The whole of the molar surface is, 

 however, not formed from the third tooth ; for, behind the second tooth, there 

 is a ridge running backwards, which fades off into a rounded prominence, 

 thus forming the rest of the molar surface. 



The second tooth therefore undergoes a transformation similar to that 

 which occurs in the third, for it becomes a V-shaped ridge, one limb of 

 which forms the outer cutting-edge of the mandible and the other the molar 

 ridge just mentioned. The bifurcate characters of the second and third 

 teeth are better seen in the early calcified stages than in the adult where, 

 however, they can readily be made out. Thus in a small crawfish 27 mm. 

 iii length, in which the mandibles only are calcified, and in a later stage 

 32 mm. in length, in which both mandibles and cuticle of the body generally 

 are calcified, these two V-shaped structures are readily distinguishable. 



(7) Antennular screen. 



Another marked change in the transition from the puerulus to the post- 

 puerulus is the disappearance of a bundle of long feathered seta?, arranged 

 as a fan-shaped structure, on the upper side of the distal end of the first 

 segment of the antennule. It is not readily seen in a surface view, as it lies 

 at right angles to the axis of the antennule, and it is obscured by the thick 

 antenna? in a lateral view. If, however, the antennules are removed and 

 viewed laterally, the setse are very obvious. There are about half a dozen of 

 them all inserted close together in a short transverse furrow, thus forming a 

 sort of screen in front of the auditory pit, which lies at the other end of the 

 same segment. The auditory pit is as yet widely open, and is protected by 



