PROSOMATIC SEGMENTS OF LIMULUS 239 



in Stylonurus, but all five possess no special locomotor or prehensile 

 power. According to Laurie this is a specially primitive form of the 

 group. 



It is significant to notice from this sketch that with the absence 

 of special prehensile terminations such as chelas, or the absence of 

 special locomotor functions such as walking or swimming, these 

 appendages tend to dwindle and become insignificant, taking up the 

 position of mere feelers round the mouth, and at the same time are 

 concentrated and pressed closely together, so that their appendage- 

 nerves must also be close together. 



This sketch therefore shows us that — 



Of the six foremost prosomatic appendages, the chelicerre and the 

 four endognaths were, at the time when the vertebrates first appeared, 

 in very many cases dwindling away ; the latter especially no longer 

 functioned as locomotor appendages, but were becoming more and 

 more mere palps or tentacles situated round the mouth, which could 

 by no possibility afford any help to locomotion. 



On the contrary, the sixth pair of appendages — the ectognaths — 

 remained powerful, being modified in many cases into large oar-like 

 limbs by which the animal propelled itself through the water. 



It is a striking coincidence that those ancient fishes, Ptericthys 

 and Bothriolepis, should have possessed a pair of large oar-like 

 appendages. 



At this time, then, in strong contrast to the endognaths, the 

 ectognaths, or sixth pair of appendages, remained strong and vigo- 

 rous. What about the seventh pair, the chilaria of Limulus ? 



Of all the prosomatic appendages these are the most interesting 

 from the point of view of my theory, for whereas in the scorpion of 

 the present day they have dwindled away and left no trace except in 

 the embryo, in the sea-scorpions of old, far from dwindling, they 

 had developed and become a much more important organ than the 

 chilaria of Limulus. 



In all these animals a peculiarly striking and unique structure is 

 found in this region known by the name of the metastoma, or lip-plate 

 (Figs. 8 and 104 (7)) ; it is universally considered to be formed by 

 the fusion of the two chilarial appendages. 



All observers are agreed that this lip-plate was freely movable. 

 Nieskowski considers that the movement of the metastoma was 

 entirely in a vertical direction, whereby the cleft which is seen 



