INTRODUCTION. 11 



terminal one; its extremity is forked, and the external 

 angles long, slender, pointed, and attached to the last 

 ann ulation by joints. Between the external angles, and 

 on each side of the median line, are three lesser spines, 

 also attached to the last ring by joints. Between the 

 eyes, and from near the edge of the shield, hangs a long, 

 stout, and somewhat compressed appendage, which, as the 

 animal moves, is reflexed posteriorly between the claws. 

 Under each eye there is also another appendage, shorter, 

 and slightly more compressed. The claws are in three 

 pairs ; each is composed of three joints, and terminates in 

 four long, slender, hair-like appendages. These claws are 

 generally bent on the body, but stand in relief from it. 

 If the animal be viewed in front, the lower margin of the 

 dorsal shield will be found to be waved into three semi- 

 circular festoons, the two external of which are occupied 

 by the eyes, and between which the middle one inter- 

 venes ; the general direction of the claws will be seen to 

 be at right angles to the body. As the young lies en- 

 closed within the membranes of the egg, the claws are 

 folded on each other, and the tail is flexed on them so far 

 as the margin of the shield, and, if long enough, is re- 

 flected over the front of the shield between the eyes. 

 The dorsal spine is bent backwards, and lies in contact 

 with the dorsal shield; for the young, when it escapes 

 from the egg, is quite soft, but it rapidly hardens and 

 solidifies by the deposition of calcareous matter, in what 

 may be called its skin. The progress of this solidifica- 

 tion may be very beautifully observed by watching the 

 circulation in the dorsal spine. When the creature has 

 just effected its liberation from the egg, the blood glo- 

 bules may be seen ascending to the apex ; but as the 



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