lxxxil PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 1 89 5, 



segments to which they are attached, exceedingly simple, yet 

 characteristic ; and with the exception of the antennae and the four 

 succeeding pairs of appendages, which are modified to serve as 

 mouth-organs (maxillae and maxillipeds), the whole series are 

 simple biramose natatory or walking-feet, such as persist still in 

 adult Mysis and many other recent Crustacea. 



The eyes in trilobites closely resemble those of other arthropods, 

 but vary somewhat in position and also in development, in some 

 genera the eyes being altogether absent, as in Ampyx, Ceraurus, etc., 

 whilst in others, like JEglina, they are enormously exaggerated in 

 size. In some genera the eyes are hyaline, the faceted surface being 

 covered with a fine transparent layer, whilst in others the facets 

 appear prominently on the surface. It is suggested by Bernard 

 that the minute pore observed in the head, near the compound 

 eye in several genera (Trinucleus, Acidaspis, Calymene, Ampyx, 

 Griffit7iid.es, Phillipsia, etc.), may be analogous to the pore in the 

 head-shield of Apus and be the opening into the water-sac covering 

 the eyes ; and whilst in some genera of trilobites this water-sac 

 may have existed, it may have degenerated in others, leaving the 

 eye in contact with the outer cuticle, which covered it like a thin 

 transparent membrane. 



In none of the trilobites have larval eye-spots been observed. 



Dr. Lang 1 held the view (in 1891) that if a fifth pair of cephalic 

 limbs were found comparable with the anterior antennae, trilobites 

 might then be regarded as primitive entomostraca, to be derived 

 from the same racial form as the phyllopoda. I venture to think 

 it highly probable that the simple multiarticulate antennae of Tri- 

 arthrus are the anterior antennae 2 and not mouth-organs, and that 

 four pairs of gnathopodites exist in addition to them. 



Walcott is of opinion that the trilobita formed a distinct 

 branch, which diverged at a very early date from the phyllopoda, 

 and having expended its vital energy in Palaeozoic times it dis- 

 appeared. He adds : ' Probably two thousand species and one 

 hundred or more genera are known from Palaeozoic strata. With 

 this great differentiation the initial vital energy of the group 

 became impaired, and the trilobita died out at the close of Palaeozoic 

 time.' He even proposes to keep the trilobita and the merostomata 



1 'Text-book of Comparative Anatomy,' English edition, 1891, p. 415. 



2 C. D. Walcott's figure (Geol. Mag. 1894, pi. viii. fig. 1) is by no means 

 conclusive as to the exact position of these simple antennas, and possibly their 

 point of attachment may be found to be in front of the mouth. But in 

 Apus, Nebalia, and also in all Nauplius larvae they are attached at the sides of 

 that structure. The point, therefore, is immaterial. 



