54 8 



CRUSTACEA. 



whether Limulus should properly be placed among the Crustacea or 

 the Arachnida. It is sufficient to note that the Xiphosura are clearly 

 related to the Eurypterids on the one hand and the Trilobites on 

 the other hand ; and that when the natural systematic arrangement 

 of the Arthropoda shall have been finally settled, these three orders 

 will necessarily be placed close together. The Xiphosura and the 

 Eurypterida, in particular, are closely connected together by means 

 of Hemiaspis and its allies. So much so is this the case, that while 

 Dr Henry Woodward places Hemiaspis and the genera related to it 

 among the Eurypterids, these forms are placed by Zittel among the 

 Xiphosurans, an arrangement which will be followed here. 



The existing species of Limulus are aquatic in habit and are 

 inhabitants of the sea. The oldest fossil types of the Xiphosura 



Fig. 411. — A, Part of a transverse section of the tail-spine of Limulus : a, Outer cuticular 

 layer; b, Intermediate finely tubulated layer; c, Internal layer of finely tubulated tissue with 

 interspersed larger tubes, b, Part of a tangential section of the inner layer of the cephalic 

 buckler ; d, Portion of the section showing the minute dots representing the transversely divided 

 tubuli, together with the cut ends of the larger tubes, which are alone shown at e. (Original.) 



{Hemiaspis, Neolimulus, &c), appear in the Silurian rocks. Other 

 ancient types of the order {Belinurus and Prestivichia) appear in the 

 Carboniferous formation. The oldest types of Limulus itself are 

 found in the Triassic rocks. 



The Xiphosura are divided by Zittel into the two families of the 

 Hemiaspidce and the Limulidce, the former including all the Palaeo- 

 zoic types of the order, while the genus Limulus alone is comprised 

 in the latter. In the Hemiaspidce, the cephalic shield is separated 

 from the thorax, and sometimes possesses a " facial suture." The 

 thorax Y is composed of five or six segments which are usually free 



1 By Packard the head-shield of the Hemiaspidce is regarded as representing 

 the entire cephalothorax ; and the free segments which immediately follow this 

 are considered as belonging to the abdomen and not to the thorax. 



