348 Proceedings of the Royal Irish Academy. 



and the degeneration of tlie compound eyes. Thus we are led to trace 

 the Arachnida (including the Merostomata and Xiphosura) back to 

 ancestors which cannot be regarded as Arachnids, but which were 

 identical with the primitive Trilobites, and near the ancestral stock 

 of the whole Crustacean class. 



According to this conception of the primeval Arachnids, they early 

 lost the Crustacean antennules ; and the primitively compound eyes 

 underwent more or less degradation. The foremost of the ordinary 

 appendages, that, in the Crustacea, became the anteimge, took on 

 in the Arachnida the function of chelicerae ; this homology is sup- 

 ported by a comparison between the antennal glands of the former 

 class and the poison or spinning-glands found on the chelicerae in 

 several orders of the latter. The Crustacean mandibles seem to be 

 represented among the Arachnida only by the palps of the Pycno- 

 gonida, while the pedipalps of typical Arachnids (ovigerous legs in 

 the Pycnogonida) and the first pair of walking-limbs correspond 

 respectively with the two pairs of Crustacean maxillae. Thus we come 

 to the hind margin of the ancient Arthropod head. (See pp. 354-5.) 



Jaworowski's interpretation ('91) of the embryonic limbs of Spiders 

 as consisting of protopodite, endopodite, and exopodite (which 

 develops into the actual leg), would support the Crustacean re- 

 lationship of the Arachnids, at the expense of the homology of the 

 walking-legs in the two classes. In Crustacea, the endopodite is the 

 ambulatory branch, and doubtless this is also the case in the 

 Arachnida. The "endopodite " of Jaworowski seems to be the basal 

 masticatory region of the appendage. 



It is strange what differences of opinion have prevailed regarding 

 the possibility of near relationship between the Trilobites and the 

 Limidoids. For example, Bernard ('94) considers such a relationship 

 unquestionable, deriving the Limuloids directly from] the Trilobites, 

 and refening both orders unhesitatingly to the Crustacea. Lankester 

 ('97) also claims a relationship between the two orders ; but he refers 

 the Trilobites, despite their antennae and biramous limbs, as well as 

 the Limuloids, to the Arachnida. Kingsley ('94) considers the 

 Limuloids akin to the Arachnida, and the Trilobites to the Crustacea, 

 denying any close relationship between the two orders. According 

 to the view here adopted, the Xiphosura and Merostomata are to be 

 referred to the Arachnida, while the antennae and biramous limbs of 

 Trilobites oblige us to class them with the Crustacea. A direct 

 descent of Limuloids from specialized Trilobites cannot be maintained, 

 since the Merostomata, which are certainly more primitive than the 



