ZOOLOGICAL POSITION OF THE TRILOBITES. 37 



gradually diminishing rudimentary appendages. The fully 

 developed Apus is, in fact, little else than an enlarged, 

 not metamorphosed, Apus Nauplius. At any stage at which 

 the growth stopped there would always be a number of 

 progressively diminishing limb-buds and undeveloped seg- 

 ments. We accordingly find in the different species of Apus 

 a very varying number of these rudimentary limbs behind 

 a certain number of well-developed appendages at the 

 anterior end of the body. 



This argument is still further strengthened when the 

 character of the limbs is also taken into account. A 

 distinct tendency towards the formation of jointed fila- 

 mentous appendages can be seen at the anterior end of 

 the trunk. This is gradually lost towards the posterior 

 end, and the limbs become more and more leaf-like and 

 primitive, i.e., they are flat unjointed skin-folds. 



Here then was an immense amount of evidence to 

 show that the immediate ancestors of Apus must have 

 had upwards of sixty well-developed segments. Apus 

 then must stand at the very bottom of the genealogical 

 tree of the modern Crustacea, with their nearlv constant 

 number of segments, twenty to twenty-one. Indeed, as 

 above stated, so low did Apus appear in the scale of 

 Crustacean organisation that it seemed to form a 

 true connecting link between the Crustacea and the 

 Annelids. 



Now this same remarkable character in the segmenta- 

 tion, its fixation at the posterior end of the body in a rudi- 

 mentary condition, which is unknown in any other living 

 Crustacean besides Apus, is visible in many Trilobites. The 

 segments taper away at the end of the body, decreasing 

 progressively in size. So feebly developed, indeed, are the 

 most posterior segments, that I am disposed to consider the 

 pygidium of the Trilobites as a modification largely due to 

 the difficulty of bending such rudimentary segments one 

 upon the other. The earliest known genus, Olenellus, has 

 the diminishing segments free to the end, and is, in this 

 respect, even more primitive than Apus. The free condi- 

 tion of the segments in Olenellus shows that the pygidium, 



