THE ARACHNIDA. 53 



J n the Crustacea (Fig. 25), the body is distinguishable 

 into a variable number of "somites," or definite segments, each 

 of which may be, and some of which always are, provided with a 

 single pair of articulated appendages. The latter proposition is 

 true of all existing Crustacea: whether it also held good of 

 the long extinct Trilobites, is a question which we have no 

 means of deciding. In most Crustacea, and, probably in all, 

 one or more pairs of appendages are so modified as to subserve 

 manducation. A pair of ganglia is primitively developed in 

 each somite, and the gullet passes between two successive pairs 

 of ganglia, as in the Annelida. 



No trace of a water-vascular system, nor of any vascular 

 system similar to that of the Annelida, is to be found in any 

 Crustacean. All Crustacea which possess definite respiratory 

 organs have branchiae, or outward processes of the wall of the 

 body, adapted for respiring air by means of water ; the terres- 

 trial Isopoda, some of which exhibit a curious rudimentary 

 representation of a tracheal system, forming no real exception 

 to this rule. When they are provided with a circulatory organ, 

 it is situated on the opposite side of the alimentary canal 

 to the principal chain of ganglia of the nervous system ; and 

 communicates, by valvular apertures, with the surrounding- 

 venous sinus — the so-called " pericardium." 



The Crustacea vary through such a wide range of organiza- 

 tion that I doubt if any other anatomical proposition, in addition 

 to those which I have mentioned, except the presence of a 

 chitinous integument and the absence of cilia, can be enunciated, 

 which shall be true of all the members of the group. 



It is this extreme elasticity, if I may so speak, of the 

 crustacean type which renders the construction of any defini- 

 tion of the Crustacea, which shall include all its members and 

 exclude the next class, the Arachnida, so difficult. For the 

 Spiders, Scorpions, Mites, and Ticks, which constitute this class, 

 possess all the characters which have been just stated to be 

 common to the Crustacea save one ; when they are provided 

 with distinct respiratory organs, in fact, these are not external 

 branchiae, adapted for breathing aerated water, but are a sort of 

 involution of the integument in the form of tracheal tubes, or 



