256 



CIRCULATION IN ARACHNIDA AND ANNELIDA. 



Fig. 144. DORSAL VESSEL 

 OP SPIDER. 



accomplished by the conveyance of air, by means of minute 

 air- tubes, into every part of the body, however small ( 321) ; 

 a mode of respiration different from 

 any that we notice elsewhere. A very 

 similar arrangement of the circulating 

 apparatus is met with in the Spider 

 tribe ; but as the body is not so long, 

 the dorsal vessel is less extended in 

 length, and is of larger diameter. This 

 is seen in fig. 144 ; where a represents 

 the abdomen of the animal; ar, the 

 large dorsal vessel or heart ; c } a trunk 

 passing forwards to the head; and v, 

 vessels communicating with the re- 

 spiratory organs. 



294. In the animals of the Worm tribe, 

 belonging to the class Annelida, there 

 is a general similarity in the course of 

 the blood to that which prevails in Insects ; but as the respi- 

 ration is accomplished by means of special organs, which are 

 sometimes diffused along the entire body, and sometimes 

 restricted to one part of it ( 314), there is considerable variety 

 in the provisions for submitting the blood to the influence of 

 the air. In those which possess red blood ( 226), this fluid 

 can be seen to move in a closed system of vessels ; whilst a 

 colourless fluid containing numerous corpuscles flows through 

 a set of canals prolonged from the general cavity of the body. 

 It may be surmised that the two principal offices to which the 

 circulation of the blood is subservient, are here separately 

 performed; the red non-corpusculated fluid having for its 

 office to aerate the tissues, whilst the colourless but corpus- 

 culated fluid serves for their nutrition. 



295. A very curious departure from the normal type of the 

 circulation presents itself in the class of TUNICATA, the lowest 

 of the Molluscous series ( 114). The heart in these animals 

 is much less perfectly formed than in the higher tribes ; 

 though it still contains two cavities, one for receiving and the 

 other for impelling the blood. The blood may be sometimes 

 seen to flow in the direction customary among Mollusks; 

 coming to the heart from the respiratory surface, and then 

 going forth through an arterial trunk that conveys it into a 



