SECT, xiv THE EURYPTERID^E 247 



limbs of the modern Crustacea we have almost every 

 possible combination of the parts of an Annelid para- 

 poclium. We have, for example, the sensory cirrus 

 alone in the antennae, the gills alone in many Crustacea 

 (e.g. Caprella), the dorsal parapodia alone in the 

 ambulatory limbs of the Decapoda, the ventral para- 

 podia alone in the mandibles, and all these parts 

 together in the typical Phyllopodan limb. Other 

 combinations, such as the dorsal parapodium with the 

 sensory cirrus, the dorsal parapodium with the gill, 

 will no doubt suggest themselves to the reader. 



What was said above as to the first trunk limb of 

 Pterygotus applies equally well to the first trunk limb 

 of Eurypterus. We may further add that their 

 form as rowing limbs is just what is required to give 

 the animals the forward darting movements which we 

 have assumed to have led to the modifications of 

 their mouth parts. Whether they kept up a continual 

 rowing motion like the common free-living Copepoda, 

 or lurked at the sea bottom to dart out in pursuit of 

 prey which happened to come within reach, it is 

 difficult to say ; we incline to the latter as the more 

 probable habit of life. 



Again, as already described, the use of the large 

 limb in the Eurypteridae throws some light on that 

 made by the Trilobites of their large first trunk 

 limb. It functioned as a kind of springing foot to 

 supplement the more deliberate method of crawling. 

 The animal kingdom supplies us with many ex- 

 amples of special arrangements for such a sudden 

 and more energetic method of locomotion, developed 



