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PROF. J. D. F. GILCHRIST ON A 



short feathered setae round its margin, so that it seems not an unreasonable 

 conjecture that this screen is for the further protection of the organ at this 

 sta^e, the large antenna? forming effective protections at the sides. Its 

 function may, however, be sensory, but, whatever it be, the structure would 

 appear to be necessary only at this stage, as it is absent in the phyllosoma, 

 and very much reduced in the post-puerulus. Fig. 13 shows the structure 

 in the shed cuticle of the puerulus and fig. 12 its reduced condition in the 

 antennule, from which it was cast off. The seta; have, in the second stage, 

 become very much shorter, so that, whereas they could reach beyond the 



Fig. 12. —Reduced anteimular screen Fig. 13. — Anteimular screen 



in post-puerulus. in puerulus. 



seg. 1, first segment of antennule ; seg. 2, second segment of antennule. 



distal end of the. second segment in the puerulus when bent forwards, they 

 now could scarcely reach to the distal end of the first segment. 



In older specimens the setse -become further reduced, though, even in the 

 largest crawfish, they are found in this position. The furrow, however, in 

 which they are lodged is well marked, though relatively shorter, being in the 

 adult about a fifth of the diameter of the anteimular segment. It is some- 

 what semilunar in shape, the concavity being directed forward. It has a 

 number of pit-like depressions for the seta?, and these are often separated by 

 marked prominences in the cuticle. In front of this semilunar furrow is a 

 shallow depression in which the seta; lie. The persistence of this organ, in 

 the adult in such a definite form, seems to suggest some function, which of 

 course cannot be, in this ease, the protection of the auditory cavity. 



