ZOOLOGICAL POSITION OF THE TRILOBITES. 47 



The Trilobites seem to have perfected the browsing 

 manner of life, spending the whole of their active life in 

 crawling over the sea bottom. The primitive dorsal 

 shield at the anterior end of the body repeated itself on 

 every subsequently developed segment in the form of a 

 pair of sharp blade-like pleurae. The Trilobites thus browsed 

 securely under a great jointed roof, each joint being 

 provided with a pair of formidable lateral spines. The 

 development of these enormous pleurae may have early 

 tended to limit the number of segments. In adaptation to 

 this settled creeping manner of life, the phyllopodan limbs 

 became early transformed into filamentous walking legs. 

 The limbs, commencing from the second pair, were used as 

 locomotory (walking) appendages [cf. Triarthrus, Limulus). 

 Only under the pygidium, in the rudimentary segments, 

 did a few of the appendages remain phyllopodan. The 

 function of raking together the food, and pushing it for- 

 wards to the mouth, must have been entirely carried on by 

 the ventral branches of the parapodia. In Limulus, we find 

 these " gnathobases " highly developed, and carrying a cirrus- 

 like appendage, while in Triarthrus (cf. Dr. Beecher's 

 figures) those nearest the mouth are plate-like jaws, while 

 those on the trunk segments have a remarkable leg-like 

 appearance which can hardly be accidental. I would like 

 to suggest that some special development of these gnatho- 

 bases might be expected in the Trilobites, that is, if the 

 dorsal leg-portions were purely locomotory. 



These are a few of the more important points of interest 

 in the specialisation of the Trilobites. The type was 

 plastic enough to give rise to endless small variations, but 

 apparently not plastic enough to re-adapt itself to certain 

 new, and at present unknown, changes in the conditions of 

 life ; hence the extinction of the race. 



The other great branch of these primitive annelidan 

 Crustacea has had a very different fate. The head shield 

 was not primitively repeated along the body as pleurae, 

 but grew backwards as a great fold, forming a 

 carapace which protected the back and sides of the 

 trunk. The trunk segments remained simply cylindrical 



