Vol. 51.] POSITION OF THE TRILOBITES. 359 



before any great specialization of the mouth-limbs took place. All 

 we can safely postulate with regard to these pleurae, judging agaiu 

 from later specializations, is that they were protective, and perhaps 

 originally only for the protection of the larvae with but few seg- 

 ments ; this might account for their limitation in many subsequent 

 forms to the head-segments alone. In the meantime the body was 

 shortening by the fixation, at its posterior end, of a certain number 

 of segments in a rudimentary condition. 



From these primitive crustacean-annelids a group branched off, 

 their specialization being the continuation of the original head- 

 pleurae along the whole length of the body. This was in adaptation 

 to a crawling manner of life, the limbs on the developed segments 

 becoming modified into legs. These, the trilobites, were, in fact, 

 as I ventured to call them three years ago, browsing armoured 

 annelids. In direct line with these arose Limulus and the Eury- 

 pterids, the last-named being specialized by the secondary degene- 

 ration of the pleurae. 



The main branch developed the pleurae primarily on five seg- 

 ments only, where they formed a head-shield, which grew back as a 

 fold over the posterior segments. The trunk-segments remained 

 cylindrical, and the trunk-limbs persisted as swimming-plates. The 

 limbs near the mouth, on the other hand, became greatly specialized, 

 the most important modification being the rejection of the second 

 appendages as jaws, leaving them free to become sensory feelers, 

 and the great development of the jaw-pieces of the third pair as 

 mandibles, working well within the mouth-aperture. The process 

 of shortening the body by the fixation of a still larger number of 

 the inherited body-segments in a larval condition became more and 

 more marked. Such a development answers to Apus, from which 

 animal all the higher Crustacea can be deduced. 



Discussion. 



The President remarked that Mr. Bernard's paper dealt with 

 some additional points of structure observed in certain trilobites 

 recently described by Dr. C. E. Beecher, and he suggested that, in 

 all probability, the very small, rudimentary, paired appendages, 

 attached to the posterior segments in Triarthrus and Trinucleus, 

 indeed even the fringes of setae on the exopodites generally (like the 

 similar setaceous appendages in My sis), may have served as branchial 

 appendages. Again, although there does not appear to have been a 

 pair of very large and specialized jaws in the trilobites, as seen in 

 Apus, we may feel certain that 2 or 3 pairs of the anterior appen- 

 dages in trilobites had already been differentiated to serve as simple 

 jaw-feet. 



He referred to the Author's important observation made in his 

 former paper as bearing on the resemblance of Apus to trilobites — ■ 

 namely, that at a certain stage a number of the hinder segments in 

 both had become fixed as a series of not fully developed and still 

 rudimentary segments. 



