2 ^Ir. R. I. Pocock on the 



pliylogeny of the Arachnida. This at least seems to be borne 

 out by tlic phylogeiictic tree of tliis group tliat was published 

 by Ur. Thorell in 1877 *. From this tree it appears that the 

 Scorpions brancli oft' from the Pedipalpi, the latter from the 

 Aranea}, the Aranere from the Opilioncs, &c., almost the 

 lowest branch of the Arachnid trunk being assigned to the 

 Acari. 



Perhaps, so far as complexity of structure is concerned, a 

 Scorpion stands higher than a sheep-tick, so, no doubt, docs 

 an active frce-swimmlng lobster rank above a sluggish Maioid 

 crab. Nevertheless no one would probably on these grounds 

 place the Maiida3 in a phylogcnetic tree at a lower level than 

 the Astacidfe. But the reasons that lead us to consider the 

 ]\racrura nearer than the Brachyura to the ancestor of the 

 Decapoda point also to the conclusion that of all Arachnids 

 the Scorpions most resemble the primitive type ; for in these 

 animals the metamerism of the body is more fully expressed 

 than in any other order of Arachnida, all the twelve somites 

 of the abdomen being well developed with dorsal and ventral 

 representatives, the anterior six of them bearing permanent 

 or transitory appendages ; furthermore, the nerve-chord is 

 furnished with a series of ganglia and the heart is divided 

 into a greater number of chambers. 



Considering, then, on these grounds the primitiveness of 

 the Scorpion's structure, we may imagine that the immediate 

 ancestor of all the Arachnida was constructed somewhat as 

 follows t : — The body was composed of eighteen somites, the 

 anterior six of which were provided with large appendages 

 set apart for locomotion and the prehension and mastication 

 of food ; the tcrga of this cephalothoracic region were fused 

 to form a single shield or carapace, supi)orting a submedian 

 and a cluster of lateral eyes on each side, and the ventral 

 surface of the carapace, at least in its posterior half, was 

 protected by a sternal plate. Each of the succeeding six 

 somites bore a pair of small ventral appendages, and the 

 generative aperture opened upon the sternal area of the first 

 of these somites. The posterior six somites had lost their 

 appendages, were probably narrower than the rest, and con- 

 stituted a limbless caudal termination to the body, the last of 

 them being furnished with a single plate, articulated above 

 the anal aperture. 



The Scorpions have de])artcd from this liypothetical type 

 in the following particulars : — The otherwise useless posterior 



• Actes Soc. Ital. Sci. Nat. xix. p. 86, 



+ Cf. in this connexion Prof. Lankester's deflnitiou of the class Arach- 

 nida in Quart. Journ. Micr. Sci. xxi. p. 647 (1881). 



