CRUSTACEA. 3 1 



older ones. The separation in the crayfish is also very 

 narrow, a mere line of uncalcified integument, visible under 

 a magnifier. By partially drying, however, and observing 

 from the inside where the calcification is less complete, the 

 plate {site I ) can be more readily observed, and the division 

 also. In this figure the line mt ends upon the uncalcified 

 bases of the metastomata, and immediately below this line, 

 where it passes the outer border of the left metastoma, is 

 the hooked-shaped calcified portion or posterior border of 

 the same, which rests upon sue \. mt i points out the 

 calcified, leaf-shaped, free upper parts of the right metas- 

 toma, the membranous part in black. The lines indicating 

 sutures between sue 2, snd 1, snd 2, and snd 3. are theo- 

 retical, indicating the author's view of the probable position 

 of the sutures ; but the line posterior to sp, the spinous 

 plate of sini 3, was actually visible in one preparation care- 

 fully cleaned. The sternal plates are composed of three, 

 and in some cases four, pairs of distinct pieces, the spine 

 or plate sp representing the posterior pair, the knobs kp 

 the next, and the bridge or beam snd may be either one or 

 two pairs, according to the segment. From s/> to ed there 

 are no visible sutures in any of the preparations I have yet 

 seen. The metastomata are not regarded as limbs by the 

 majority of naturalists, but this view is not justified by the 

 facts. They are wholly separated from the bases of the first 

 pair of maxillae, and are independent outgrowths or buds 

 from the integument, as much as any other pair of append- 

 ages : and the fact that the parts of the segment to which 

 they must have belonged have disappeared, or cannot be 

 readily found, is an argument of doubtful weight. It is 

 difficult to account for them unless liny are acknowledged 

 to be the remnants of a pair of true appendages which have 

 become reduced to their present stale by change of func- 

 tion or disuse. This view, however, cannot be maintained 

 without making this pamphlet too scientific, and therefore 

 not so useful to teachers. 



