4 6 SCIENCE PROGRESS. 



fates — the Trilobites to die out with a single survivor in 

 Limulus, 1 Apus not only to survive to this day, but to give 

 rise to the whole family of modern Crustacea. 



Before describing the lines along which the two forms 

 must have diverged, one other very early specialisation of 

 the common racial form has to be mentioned. The bending 

 round of the prostomium with the antennae, and of the para- 

 podia with their protective bristles, towards the ventral 

 surface, left the dorsal surface quite exposed ; some com- 

 pensatory arrangement for protection was therefore 

 needed. It is not improbable that the dorsal organ, which 

 in the larva is comparatively of great size, and forms a 

 shield-like plate of excretory cells, was protective. In 

 addition to this, however, the skin of the dorsal surface 

 at a very early stage not only thickened, but grew out 

 laterally into folds which roofed the animal over, while 

 the skin of the under surface retained its primitive delicacy 

 and softness, hence its almost universal absence in the fossil 

 Trilobites. The earliest arrangement of these dorsal 

 shields must for the present remain matter of conjecture. 

 The very earliest stage, both ontogenetic and phylogenetic, 

 known to us is that presented by the minute larva of Olen- 

 ellus, the most primitive Trilobite. We there find an 

 enormous skin-fold forming a crescent round the front 

 and sides of the first segment, the three following seg- 

 ments having much smaller lateral folds (pleurae), which, 

 fusing with this crescent fold of the first segment, together 

 form a roof almost completely protecting the young Olen- 

 elhis." There is some reason to believe that primitively 

 this dorsal roof was confined to the anterior end of the 

 body covering the first five segments. From this common 

 stage the Trilobites, and the Crustacea proper, parted 

 company. 



1 I am inclined to believe that the Ostracoda also may be deduced from 

 larval Trilobites in which the head shield folded longitudinally to form the 

 bivalve shell {cf. The Apodidee, p. 256). 



2 From such extremely primitive crustacean or rather pre-crustacean 

 larvae with their great circular roofs, I would deduce the Ostracoda, and 

 the enigmatical palaeozoic Cyclus. 



