ANNULOSA: AEACHNIDA. 197 



8. Stomapoda.TlnB order is doubtfully represented in the 

 Carboniferous Bocks. 



9. Decapoda. The Decapods are not known to have existed 

 at all during the Palaeozoic period ; but they are well repre- 

 sented, in all their three tribes, in the Secondary and Tertiary 

 Epochs, attaining their maximum at the present day. ^ The 

 London Clay (Eocene) is especially rich in the remains of 

 Macrura and Brachyura. 



CHAPTER XXXYI. 

 AEACHNIDA. 



CLASS II. AEACHNIDA. The Araclimda including the Spiders, 

 Scorpions, Mites, &c. possess almost all the essential cha- 

 racters 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 appen- 

 dages. 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 vehicles 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 Araclinida, as in the higher Crustacea, 

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

 allotted 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 antennas 

 are developed.' (Huxley.) 



The head in the Araclinida 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 Araclinida the integument remains soft over 



