494 THE ANIMAL KINGDOM. 







on rhythmic motions of the intestine (e.g. some Copepoda among Crustacea}. 

 A contractile heart, composed of a single chamber or many chambers, each 

 with its pair of ostia, is generally present, and lies dorsally. The lips of 

 the ostia form internal valves which prevent regurgitation. The heart is 

 always open anteriorly, and, except in Insecta and Myriapoda, as a rule 

 posteriorly also. The extent to which arterial capillary and venous 

 channels are differentiated varies, but it is slight except in Myriapoda, and 

 especially in Scorpio and Limulus, and the highest Crustacea. A peri- 

 cardial sinus exists in the higher Crustacea, Arachnida, the Myriapoda and 

 Insecta. In the three classes last named, it is dilated by a system of 

 segmentally arranged alary muscles, which affect the heart only indirectly. 

 The blood currents flow in constant directions. 



Respiration may be cutaneous, i.e. effected by the general surface of 

 the body, in the lower Crustacea and Arachnida ; or branchiate, i.e. effected 

 by processes developed from the surface of the body, or in connection with 

 the limbs, as in most Crustacea and some Arachnida ; or tracheate, and 

 carrying air to all the tissues, as in Insecta, Myriapoda, most Arachnida, 

 and the Protracheata. The tracheae are developed as internal growths 

 from the hypodermis, lined by a chitinoid cuticle, which is generally 

 crenulated spirally in the tracheal tubes. The simplest form is seen in 

 Protracheata as a depression of the integument, from which arises a bundle 

 of tracheal tubes. The structure is much the same in Myriapoda Diplo- 

 poda. But as a rule the tracheal tubes branch and anastomose. The 

 external apertures are known as stigmata. In certain Arachnida (e.g. 

 Scorpio) the depression is large, and its walls disposed in leaf-like folds, 

 tracheal tubes being absent. Such structures form the so-called lungs or 

 lung-books. The tracheal system may become closed externally in aquatic 

 forms, and then external processes of the integument, variously shaped, 

 may be developed, in which tracheal branches ramify, as in the tracheal 

 gills of some Insecta. Air enters and is expelled from the open tracheal 

 system by contractions of the body walls and by diffusion. Peripatus has 

 numerous tracheal pits ; in other tracheate Arthropoda more or fewer of 

 the somites possess each a pair of stigmata, the head alone excepted, on 

 which they are very rarely present. It has been suggested that the tracheae 

 represent modified glands (Moseley). 



Excretory organs are present in Crustacea as (i) the two shell-glands, 

 (3) the two antennary or green glands, (3) caeca of the mesenteron in 

 Amphipoda, or of the proctodaeum in Squilla, whilst (4) in some instances 

 the walls of the proctodaeum itself appear to be excretory. In tracheate 

 Arthropoda the organs in question are tubular outgrowths of the procto- 

 daeum known as Malpighian tubes, varying in number, shape, &c. Uric 

 acid appears to be constantly found among the excretory products 1 . 



1 The coxal glands of Arachnida are possibly aborted excretory organs. 



