288 



THE INVERTEBRATA 



in the anterior region of the head. Traces of a groove which exist in 

 some species may perhaps indicate its existence. 



The remaining Umbs are all of one type, though there is a gradual 

 progressive modification from one end of the series to the other. Each 

 has two rami. Of these, one, usually held to be the outer ("exopo- 

 dite "), bears a long fringe of bristles, while the other (" endopodite ") 

 is leg-like and divided into six joints. It is supposed by some authori- 

 ties that the bristle fringe on the so-called exopodite was on the inner 

 side of the limb, and was used for collecting food, Hke the fringes on 



Fig. 209. Triarthrus becki, from the Utica Slate (Ordovician) near Rome, 

 New York. After Beecher. A, View of the ventral surface showing append- 

 ages, etc. h, hypostome (labrum); m, metastoma. x f. B, Diagrammatic 

 section through the second thoracic somite, a, " endopodite " ; ^, " exopodite ". 

 C, Dorsal view of second thoracic leg. «, "endopodite"; b, "exopodite"; 

 Cy " protopodite " with gnathobase. Enlarged. 



the trunk limbs of branchiopoda (p. 317). From the basal portion of 

 the limb a process for the manipulation of food, the gnathobase, pro- 

 jects towards the middle line. The configuration of the basal portion 

 (protopodite), and the relation of the rami to it, are obscure. The 

 telson is without limbs. 



The Trilobita hatched as a larva, the Protaspis, which was sub- 

 circular, and consisted, like the Nauplius larva of the Crustacea, 

 principally of head. In its further development there appear, first the 

 pygidium, and then free somites between the pygidium and the head. 



