44 Traces of Unity in the Appendicular 



fleshy suckers. In the butterfly the mandibles are quite 

 rudimentary, and the maxillae have coalesced so as to 

 form the long proboscis, or antlia, which when not in use 

 coils up spirally between two lateral palps, which seem 

 to be maxillary palps. Here, no doubt, it is difficult to 

 be always positive as to the homology of the parts. In 

 fact, the antlia may be a highly developed tongue, akin, 

 perhaps, to the proboscis of many gasteropods ; but 

 generally, there is no difficulty in passing step by step 

 from one form of mouth to another, and in tracing 

 everywhere the presence of the same elements. 



On comparing the oral appendages of a mandibulate 

 arachnidan with those of a mandibulate insect the 

 resemblances out-number the differences, and the differ- 

 ences only serve to bring out more clearly the limb- 

 nature of the different parts. The mandible of the 

 scorpion with its terminal pincers, or the mandible of 

 the spider, with its sharp, moveable fang, perforated by 

 a poison duct, point plainly to the jointed foot-jaws of 

 the crustacean ; and so, with still greater plainness, does 

 the enormous cheliferous palp upon each of the maxillae 

 an organ which closely copies the chela of the lobster, 

 not only in shape, but also in relative proportions, its 

 size being as much in excess of that of the true limbs as 

 in the crustacean. Nor is it otherwise when this com- 

 parison is made to include arachnidans with haustellate 

 mouths, for, with or without the key met with in insects 

 having mouths of the same sort, it is easy to see that the 

 oral appendages here are merely modifications of those 

 which enter into the formation of the mandibulate mouth. 



The mouth of the annulata may take the form of a 

 sucker, as in the leech, or of a proboscis, which is really 

 a protrusible and retractile pharynx, as in the arenicola, 

 or sand-lug, with or without three or more teeth of one 



