40 COMPARATIVE ANATOMY. 



beyond it, and serves as a prehensile organ. They are usu- 

 ally curved like hooks, and denticulated upon their concave 

 sides. They vary in number from two to nine pair; when 

 numerous they are of dissimilar forms with the same indi- 

 vidual. (Siebold.) They move laterally upon each other, ex- 

 cept in Acoetes. (Stimpson.) Single siliceous tooth of Bor- 

 lasia, placed in median line of head, with remote formative 

 sacs. For Hirudo (leech), see page 159. 



The parts entering into the construction of the mouth in 

 arthropod articulates, excepting, perhaps, certain Crustacea, 

 are as follows, named from above downwards: labrum (upper 

 lip), mandibles (first pair of jaws), maxillae (second pair of 

 jaws), labium (lower lip), with ligula (tongue). The palpi are 

 always appendages to either the maxillae or labium. 



Arachnida. The* mandibles (cheliceres) move horizon- 

 tally in Scorpionidae (scorpions), vertically in Araneidae (spi- 

 ders); maxillae and maxillary palpi more or less pronounced, 

 latter in male spiders employed as feelers, and in generative 

 act (see p. 116). Ligula present in Scorpionidae, absent in 

 Araneidae. Labium generally wanting; in Scorpionidae 

 deeply cleft; labial palp large, commonly progressive, used 

 as feet, but in Thdyphonus and Phrynus as antennae. (Wood.) 



Myriapoda. Organs arranged upon three types according 

 as animals are (1) carnivorous, (2) herbivorous, or (3) suctorial. 



(1) In Syngnatha (centipedes proper), labrum commonly 

 large; maxillae robust; maxillary palpi well defined with 

 ligula placed between their bases. Labium rudimentary, 

 but labial palpi large. Scutum of sub-cephalic segment or 

 ring transformed into a broad false labium, or gular plate, 

 with the feet transformed into powerful, prehensile organs, 

 and provided with poison glands. 



(2) In Chilognatha manucentia, as in Julidae (millipedes), 

 between a massive labrum and a broad opposing gular plate, 

 robust maxillae are inserted ; other organs wanting. (3) In 

 Chilognatha sugentia as in Octoglena, organs fused to form a 

 suctorial tube. (Wood.) 



Insecta. Organs arranged upon two great types (1) Man- 

 ducatory, and (2) Suctorial. 



