516 



COMPARATIVE ANATOMY 



CHAP. 



extremities is differently shaped in the Pedipalpi ; it is long and thin, 

 with long flagellate ringed terminal joints. Here it is principally or 

 exclusively used as an organ of touch. 



The Fourth, Fifth and Sixth Pairs of Extremities are, as a rule, 

 similarly shaped 6- jointed locomotory organs. In the Scorpionidce 

 the fourth pair of extremities also carries a masticatory ridge. 



In the Linguatulidce, which are regarded as Arachnoidea degraded 

 by parasitism, the limbs are reduced in number and form. Only two 

 pairs of clinging hooks are found near the mouth. Definite data for a 

 comparison of these clinging hooks with any special pairs of the limbs 

 of typical Arachnoidea are, however, wanting. 



The homologies of the Arachnoidean limbs with those of other Arthropoda are 

 difficult to establish. If we compare the Arachnoidea with the Antennata, and 

 especially the cephalo-thorax of the former with the head and 3 anterior trunk 

 segments (thorax) of the latter, we find that the Arachnoidea possess one pair of 

 extremities less than the Antennata in the corresponding regions. 



In the Sotyugidce, in which the section of the body which corresponds with the 

 cephalo-thorax of other Arachnoidea is, as in the Antennata, segmented, the head 

 carries 3 pairs of extremities, viz. the chelicerae, pedipalps, and the pair of limbs 

 which follow these. Each of the thoracic segments following the head possesses a 

 pair of limbs. Since the Antennata carry typically 4 pairs of limbs on the head, 

 viz., the antennae, mandibles, anterior and posterior maxillae, it follows presup- 

 posing that the head of the Solpugidce, really corresponds with that of the Antennata 

 that the wanting limbs belong to the head. Various facts, chiefly ontogenetic, 

 make it probable that it is the antennae of the Antennata which are wanting in the 

 Solpugidce and in the Arachnoidea generally, 1 while the other limbs correspond accord- 

 ing to their order of succession. In this way we reach the following homologies 

 between the Arachnoidean and Antennatan limbs. 



Head 



Antennata Arachnoidea 



I Antennae wanting 2 



1 Mandibles = Chelicerse 



1 Anterior Maxillae = Pedipalps 



( Posterior Maxillse = 3d pair of limbs 



Head 

 of the 

 Solpugidce 



first "j Trunk foot = fourth ^j Pair "j 3 thoracic 

 second |- (thoracic legs = fifth Y of >- segments of 

 third J of Insecta) = sixth J Limbs J the Solpugidce 



Cephalo- 

 thorax 

 of the 

 Arach- 

 noidea 



3 Anterior 



trunk 

 segments 

 = thorax 

 of Insecta 



If these homologies are correct, then the chelicerse especially, but also the pedi- 

 palps and the 3d pair of extremities of the Arachnoidea, differ very greatly from the 

 corresponding cephalic limbs of the Antennata, i.e. the mandibles and anterior and 

 posterior maxillae. The mandibles of the Antennata are never jointed and the 

 maxillae never elongated like legs, as in the Arachnoidea. Now since it is not con- 

 ceivable that the 3 anterior pairs of much-jointed limbs of the Arachnoidea have 

 proceeded from the reduced and specialised oral appendages of the Antennata, we are 

 compelled to assume that if there is any near relationship between the two groups, 



1 While these sheets are passing through the press it is announced that the embryo 

 of a large spider (Trochosa singoriensis, Laxm. ) shows distinct rudiments of antennae 

 which disappear later ; and further, that more than 4 pairs of rudimentary abdominal 

 limbs are visible, with traces of several pairs of stigmata. (Zool. Anzeiger, llth May 

 1891.) [TB.] 2 Ibid. 



