256 CIRCULATION IN ARACHNIDA AND ANNELIDA. 
Fig. 144. —Dorsal Vessel 
of 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 theWorm 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 
