cxviii Introduction. 



rather than an ahorescent arrangement, and they not rarely differ 

 also from those seen in Insecta and Myriopoda, by not possessing 

 the internal spiral thickening, so characteristic of the respiratory 

 tubes in those two classes. 



The limits within which the variations of the circulatory and ner- 

 vous systems may range, are very wide ; the arteries and veins may 

 attain, as in Scorpions, a very high grade of evolution and distinct- 

 ness, or both sets of vessels and the heart also may be absent, as in 

 such lower forms as the Acarina and Lingnatulina, in which the 

 nerve-system is reduced to a single ganglionic mass perforated by 

 the oesophagus. The supra-oesophageal and the sub-oesophageal 

 ganglia are always closely approximated in Arachnida, in corre- 

 spondence with the assignment of the antennary to act in aid of 

 the manducatory organs, and with the more or less suctorial modi- 

 fication of their carnivorous habits, with which a small pharynx 

 and oesophagus are correlated. 



The Arachnida, with the exception of the Tardigrada,2ir& dioecious ; 

 and with the exception of the order just named, and the Linguatu- 

 lina, the segmentation of the yolk is partial in the class. With the 

 exceptions of Scorpionidae and some Acarina, the Arachnida are 

 oviparous. Most Arachnida, when hatched, resemble the adult 

 animal ; amongst the Acarina, however, we meet with hexapod 

 larvae, which attain their fourth pair of legs subsequently by moult- 

 ing, whilst the Linguatulina reverse this history in their develop- 

 ment, and undergo a retrograde metamorphosis from a larval form 

 provided with two pairs of biarticulate and unguiculate appendages 

 to a vermiform adult destitute of limbs. 



Class, Crustacea. 



Water-breathing Arthropoda, which may, in accordance with the 

 grade of specialization attained to by their segments and appendages, 

 be as markedly heteronomous as the Insecta, or as homonomous ex- 

 ternally as the Myriopoda. Normally, every segment in the Crus- 

 tacean body carries a pair of articulated appendages; and two pairs 

 of antennae are all but invariably present, indicating the presence of 

 two cephalic segments between the jaws and the eyes. The append- 

 ages of one or more of the three segments immediately posterior to 

 the jaws, are always converted into auxiliary jaws, or ' maxillipedes;' 



