ZOOLOGICAL POSITION OF THE TRILOBITES. 41 



elaborated elsewhere, I may mention a few points, e.g., 

 the sharp anterior bend in the intestinal canal ; the curious 

 clumping of the ventral longitudinal muscles, which form 

 the "cartilaginous" endosternites in the angle of the bend ; 

 the corresponding stretching out of the longitudinal 

 muscles on the dorsal side ; the simple explanation this 

 bending round of head segment affords of the translocation 

 of two pairs of limbs, originally post-oral, into the pre-oral 

 position occupied by the antennae in the Crustacea ; and 

 lastly, the first ontogenetic appearance of the eyes in Apus in 

 a more anterior and ventral position than they possess in the 

 adult, and their subsequent wandering upwards on to the 

 dorsal surface. In the discussion of the remaining points 

 which we shall select for mention in the following pages, we 

 shall, therefore, assume that the organisation of these early 

 Crustacea, Apus and the Trilobites, can be deduced from 

 that of an Annelid with the first segment bent round 

 ventrally, so that the large prostomium points backwards, 

 while the pair of appendages, specialised into sensory cirri, 

 which were arranged one on each side of it in its original 

 anterior position, remained on each side of it in its new 

 ventral position, and became the first pair of antennae of 

 the Crustacea. In making this assumption, we are in no 

 sense rushing ahead of our facts ; not only does the evidence 

 already brought forward justify us, but the points still to 

 be dealt with confirm it in every instance. We state it 

 thus definitely at this point chiefly in order to facilitate 

 the comprehension of other details. 



Having mentioned the large labrum which has long 

 been known, and the antennae attached at each side of it, 

 the structure next in order, from before backwards, is the 

 under lip, which has recently been described by Dr. 

 Beecher for the first time, as it appears in Triartkrus. 



The under lip is simply the ventral border of the mouth, 

 but in Triarthrus it is bent round as a small ridge project- 

 ing backwards. In the original annelidan ancestor, with its 

 anteriorly placed mouth, the dorsal border was formed by 

 the large Meshy labrum or prostomium, while the ventral 

 border, in all probability, showed no specialisation whatever. 



