Organs of Invertebrate Animals. 51 



peduncle by which the animal is fixed, as by a root, 

 when its period of juvenile freedom is at an end a 

 change by which this particular part of the larval 

 barnacle is made to move many steps towards the 

 position occupied by the byssus-forming foot of the 

 common mussel and by the spinnaret of the spider. 



7. If there were room it would not be difficult to 

 muster many other facts of like significance : as it is, I 

 must content myself by singling out certain external 

 appendages of the prawn which can hardly be over- 

 looked, namely, the central spike projecting from the 

 anterior edge of the carapace, the tail, and the external 

 genital organs. 



The spiny-edged, lamelliform, immoveable append- 

 age which projects from the anterior edge of the cara- 

 pace in the prawn, may, for anything that appears to the 

 contrary, be homologous to any one of the limbs which 

 lie at its side may be, in fact, nothing less than one of 

 these limbs aborted in the state of spine. It lies among 

 these limbs like one of themselves, and unless it were 

 sought out it might be confounded with them. And, in 

 short, it is difficult to reject this claim to relationship 

 difficult to avoid the conclusion that the anterior part of 

 the trunk may take upon itself the form of a limb so far 

 as to be confounded with it. 



The tail, too, would seem to show still more plainly 

 that the limb-nature which may lie hidden in the 

 anterior end of the trunk is also present in the posterior 

 end. The tail of the prawn is to all intents and pur- 

 poses a large and strong limb : and more emphatically 

 still is it so in the poison-fanged tail of the scorpion. 

 In the latter case, indeed, the limb and tail agree very 

 much in dimensions, and their other differences are not 

 at all irreconcilable. But this is only a hint in passing. 



E 2 



