AXXULOSA: ARACHNIDA. 221 



i 



CHAPTER XXXVI. 



ARACHNIDA. 



CLASS II. ARACHNIDA. The Arachnida including the Spi- 

 ders, Scorpions, Mites, &c. possess almost all the essential 

 characters of the Crustacea, to which they are very closely 

 allied. Thus, the body is divided into a variable number of 

 somites, some of which are always provided with articulated 

 appendages. A pair of ganglia is primitively developed in 

 each somite, and the neural system is placed ventrally. The 

 heart, when present, is always situated on the opposite side of 

 the alimentary canal to the chain of ganglia. The respiratory 

 organs, however, whenever these are differentiated, are never 

 in the form of branchiae as in the Crustacea, but are in the form 

 either of pulmonary vesicles or sacs, or of ramified tubes, 

 formed by an involution of the integument, and fitted for 

 breathing air directly. Further, there are never " more than 

 four pairs of locomotive limbs, and the somites of the abdomen, 

 even when these are well developed, are never provided with 

 limbs;" the reverse being the case amongst the Crustacea. 

 Lastly, " in the higher Arachnida, as in the higher Crustacea, 

 the body is composed of twenty somites, six of which are al- 

 lotted to the head ; but, in the former class, one of the two 

 normal pairs of antennae is never developed, and the eyes are 

 always sessile ; while, in the higher Crustacea, the eyes are 

 mounted upon movable peduncles, and both pairs of antennae 

 are developed." (Huxley.) 



The head in the Arachnida is always amalgamated with the 

 thorax, to form a " cephalothorax ; " the integument is usually 

 chitinous, and the locomotive limbs are mostly similar in form 

 to those of insects, and are usually terminated by two hooks. 



In many of the Arachnida the integument remains soft over 

 the entire body ; in others, as in the majority of Spiders, the 

 abdomen remains soft and flexible, whilst the cephalothorax is 

 more or less hard and chitinous ; in the Scorpions, again, the 

 integument over the whole body forms a strong chitinous shelL 



The typical somite of the Arachnida is constituted upon 

 exactly the same plan as that of the Crustacea, consisting 

 essentially of a dorsal and ventral arc ; the former composed 

 of a central piece, or "tergum," and of two lateral pieces, or 

 " epimera ; " whilst the latter is made up of a median " ster- 

 num " and of two lateral " episterna." 



As regards the composition of the cephalothorax of Spiders, 



