ORGANS OF SUPPORT, 



XII. Arachnida. In those air-breathing entomoid ani- 

 mals, without metamorphosis, without wings, without an- 

 tennae, and with generally more than three pairs of legs, 

 which compose the class of aracknida, we observe a more 

 concentrated form of the segments, and a more consolid- 

 ated condition of the skeleton on the anterior portion of 

 the trunk than in the lower articulated classes. The seg- 

 ments of the head are anchylosed to those of the thorax, 

 so as to form a single division^ the cephalo-thorax, which 

 supports the organs of the senses, those of mastication, 

 and those of locomotion. The posterior division of the 

 trunk is the abdomen. In place of the long jointed antennee 

 on the anterior part of the head which we see in other 

 entomoid classes, there are generally a pair of lateral 

 pincers, or cheli, or a pair of flat and sharp-pointed piercing in- 

 struments at the sides of the head more suited to their 

 retired, cunning, watchful and carniverous habits. Their 

 mouth is provided with a labrum, a labium, a pair of man- 

 dibles, a pair of maxillae, and a pair of jointed palpi, often 

 extended like short feet. The general disposition and 

 form of the external parts in the animals of this class 

 are seen in this outline of the lycosa tarentula, (Fig. 17-) 

 where a represents 

 the cephalothorax, 

 and b the abdominal 

 portion of the trunk. 

 The eyes are simple 

 small isolated ocelli, 

 placed on the upper 

 and anterior portion 

 of the cephalotho- 

 rax, and varying in 

 number from two to 

 twelve ; they are of 

 very different sizes 

 in the same animal, 

 and variously dis- 

 posed in the differ- 

 ent species. Four 

 large eyes are seen at d, placed as at the angles of a square, 

 and four smaller eyes are seen behind them at d*, arranged 



FIG 



