Carpenter — BehitiomMps between Classes of Arthropoda. 349 



Limuloids, show less than they a superficial resemblance to Trilobitcs. 

 But though the cephalothoracic carapace of Limulus does not 

 correspond with the head shield of a Trilobite, the former may, in all 

 probability, have arisen by the fusion of three trunk-segments with 

 the primitive head-shield of the Proto-Trilobita. There is no difficulty 

 in tracing back the Merostomata, the Xiphosiu-a, and the Trilobita to 

 a common ancestry ; and thus the Axachnida as a class, like the 

 Insecta, have been evolved from Crustaceans. Except among the 

 Pycnogonida, where the reduction of the abdomen has necessitated a 

 shifting forwards, the genital openings in the Arachnida are only one 

 segment in front of the position of the female genital openings in the 

 typical Crustacea. 



The sessile eye of the Trilobites, the median telson in their more 

 primitive genera (contrasting with the tail-furca of the Leptostraca 

 and Phyllopoda), and the trilobite-larva of Limulus, all suggest the 

 probability that the Arachnida arose from the base of the Trilobitan 

 branch, rather than from the main Crustacean stem. And the loss of 

 the antennules, together with other specialized characters, shows that 

 the Arachnida have diverged much more widely from the Crustacea 

 than the lowest Insecta have. 



The Ancestry of the Arthropoda as a whole. 



"We have seen that the Insecta, the Chilopoda, the Diplopoda, the 

 Crustacea, and the Ai'achnida can all be traced back to common 

 Arthi'opodan ancestors, with a definite number of segments. The 

 origin of these primitive Arthropods now demands consideration. 

 They were distinctly Crustacean in character, so the question of their 

 history may practically be reduced to that of the very remote 

 ancestry of the Crustacea. Two theories on this subject need to be 

 discussed — the older view that the Nauplius larva represents the 

 primitive Crustacean ; and the newer, according to which the Crustacea 

 must be directly derived from Annelidan ancestors, the nauplius being 

 regarded merely as a modified trochophore with certain adult 

 Crustacean characters precociously developed. 



As an introduction to the examination of the Annelidan theory, we 

 must try to ascertain the relationships of the Malacopoda (Peripatidae), 

 since they show more Annelidan characters than any other group that 

 can be considered as belonging to the Arthropoda. Although the 

 Malacopoda have not jointed limbs, there need be no hesitation in 



