ARACHNIDA. 523 



A well-developed chitinoid piece the camerostome (the hypostoma 

 so-called, in TrilobitaT) overhangs the mouth anteriorly. The sternal 

 region in the thorax is variously developed or suppressed in the different 

 groups. The posterior part or metasternite developes in Limulus into the 

 two lobes or chilaria which lie behind the last pair of cephalothoracic limbs. 



In the tracheate forms the appendages of the cephalothorax are as 

 follows : a pair of falces or chelicerae post-oral in development, prae-oral 

 in the adult, composed of i to 3 joints, and terminated by a claw or 

 pincer-like chela (Scorpio, &c.)or else represented by a pair of stilets (some 

 Acarina] ; a pair of palpi (pedipalpi) or chelae usually large, the basal joints 

 of which are well developed, inclosing the mouth (and stilets when present), 

 and terminated either by a claw or chela, or by a copulatory apparatus in 

 the male Araneid x ; four pairs of limbs, generally elongated, ambulatory, 

 and terminated by claws, to which adhesive lobes or discs may be added 

 in some Acarina 2 . The number of joints in the limbs is variable, usually 

 6-7 ; and there may be a well-formed tarsus (Solifugae). The coxal 

 joints of the two first pairs of ambulatory limbs are in relation with the 

 mouth in Scorpionidae and Phalangida. The abdomen has appendages 

 only in Scorpionidae, the first pair forming a minute genital operculum, 

 the second the pectines or combs, organs probably of touch. The 

 remaining six pairs of abdominal limbs present in the embryo Scorpion 

 abort. Abdominal limbs are present to the number of four pairs in the 

 embryo Spider, but abort at an early period. 



Among branchiate forms Limulus has a pair of 3-jointed prae-oral 

 chelicerae, the Etirypterina homologous appendages which are sometimes 

 non-chelate. Limulus has five pairs of post-oral cephalothoracic limbs, 

 the Eurypterina and Trilobita four pairs, of which the last are the most 

 powerful. The coxae of these limbs surround the oral depression, and are 

 therefore masticatory. Limulus has six pairs of limbs on the abdominal 

 mesosoma, of which the first pair fuse to form a genital operculum, as in 

 Scorpio ; whilst the remaining five are lamellate and carry externally on 

 their posterior surfaces a series of close-set branchial folds. The two first 



1 During the development of some Acarina the embryonic chelicerae and pedipalpi fuse into a 

 proboscis. At a later stage, after an ecdysis, the stilets and palpi grow out from the proboscis, and 

 are presumed to be the homologues respectively of the chelicerae (or mandibles as they are some- 

 times termed in the Acarina) and pedipalpi. Haller states that the Acarina possess four pairs of 

 mouth-parts, a pair of mandibles, a first and a second pair of maxillae, the second often rudimentary, 

 and a labium usually palpate. As an adult Mite also possesses four pairs of ambulatory limbs, the 

 last of which Haller regards as abdominal, it has two pairs of limbs in addition to those present in a 

 Spider. A Spider however has embryonic limbs, lost in development ; and rudiments of two extra 

 pairs are present in the Scorpion, as the operculum and pectinated appendages. See pp 496-7 ante, 

 and table, pp. 174-5. 



2 Kramer holds, as do some other authorities, that the two last pairs of limbs belong to the 

 abdomen in Acarina. See his paper on ' Segmentation of the Mites,' A. N. 48 (i), 1882 (A. N. H. 

 (5), x). The abdomen in the Mite in question (Alycus roscus ?) has nine somites. 



