408 THE AMERICAN NATURALIST [Vol. LIV 



Some progress has been made toward the connection 

 of the trilobites with the Merostomata, as the Limnlava 

 of Walcott are, in a certain sense, intermediate between 

 the two. The Limulava are, however, true crustaceans, 

 for they have five pairs of cephalic appendages, the first 

 of which are elongate uniramous antennules, and also bi- 

 ramous, trilobite-like limbs on the anterior part of the 

 thorax. They are especially trilobite-like in the fact that 

 the antennae are not elongate tactile organs, but, in Emer- 

 aldella at least, are biramous. The relationship to the 

 Merostomata is expressed in the shape of the head, body 

 and telson, and the grouping of the cephalic appendages 

 about the mouth. If these animals lost the antennules, 

 developed the antennae into chelicerae, added two thoracic 

 segments to the cephalon, modified the appendages, and 

 added a sternal operculum, a merostome would be pro- 

 duced. I think it is obvious that to change a trilobite 

 into a marine arachnid is a more complicated process 

 than to change one into a crustacean of any kind. 



To compare the spiders directly with the trilobites may 

 seem somewhat fanciful, yet in some respects the spiders 

 are more trilobite-like than Limulus is. On the germ 

 band there is a pair of buds in front of the mouth which 

 probably are antennules. These later fuse to form the 

 rostrum, and the chelicerae move into a pre-oral position. 

 Moreover, Jaworowski has shown that the pedipalps on 

 the germ band of Trochosa singoriensis are biramous. 

 In young spiders the abdomen is segmented, and the an- 

 terior segments bear pairs of limb-buds, some of which 

 are later lost, while others develop into lung-books or 

 spinnerets. The number of abdominal segments appears 

 to be variable, from eight to fourteen, another feature 

 which suggests the trilobites. The spiders very probably 

 did not spring directly from this group, but will eventu- 

 ally be traced to it, and not through the Xiphosura. 



