280 COMPARATIVE ANATOMY. 



The localisation of the respiratory function leads to complication 

 of the vessels : even where the vessels are not provided with separate 

 walls the blood always flows in a definite and constant direction. 



The blood-fluid of the Arthropoda is, as a rule, colourless; in 

 a few Insects only it has a greenish, or red colour, owing to the 

 coloration of the plasma. The formed constituents of the blood 

 are indifferent colourless cells, very variable in form and size. They 

 are wanting in many (lower Crustacea). The blood-cells of Insects 

 are often distinguished by the large number of fine fat-molecules in 

 them ; but these must not be confounded with the cells of the fatty 

 bodies, which are often also free. 



§ 217. 



The simplest form of a circulatory apparatus in the Brauchiata 

 is that of a short tubular heart (cf. Fig. 136, c, of Daplmia), which is 

 placed above the enteric canal in the anterior part of the body, takes 

 iu the blood by two lateral orifices, and drives it by a short anterior 

 vessel to the cephalic organs, and especially to the cerebral ganglia. 

 The blood is distributed in regular currents throughout the body, 

 and, passing by the parts which serve chiefly as respiratory organs, 

 again reaches the heart, which it enters by the slit-like orifices in it. 

 This form of circulatory organ characterises the Copepoda and 

 Cladocera; but it is also seen in the larval stages of the higher 

 orders, and even in the developmental stages of the Decapoda, 

 where it is but slightly modified. The simple forms cannot there- 

 fore bo regarded as being degenerated from more complicated forms. 

 The circulation is purely lacunar, and there arc no vessels of any 

 kind except those added on to an anterior artery, which is seldom 

 much branched. In many Copepoda (Corycajidaj), and in the 

 Cirripedia, there are no circulatory organs. 



The heart of the Phyllopoda is more developed. It has the 

 form of a longer tube, made up by several such simple hearts as 

 those of the Daphnida, and possessing a number of venous ostia 

 (as many as 20 pairs in Artcmia). The cardiac tube, that is, is 

 divided into separate chambers, which do not, however, exactly 

 correspond to the metameres, of which there are more than one to 

 each chamber. This segmentation seems, therefore, to be an inde- 

 pendent one, and is, perhaps, to be regarded as a later arrangement. 

 An arterial trunk is given off from the most anterior end only. This 

 passes the blood on to the lacunar tract of the ccelom. 



The heart of the Arthrostraca extends longitudinally along a 

 large portion of the body in the Amphipoda and Isopoda; in the 

 former it is placed in the metameres behind the head, and in the 

 litter some way farther back. There may be an anterior vessel only, 

 or it may give off a posterior one also. The former only is branched, 

 and the branches are confined to the cephalic region. The number 

 of the ostia varies greatly in the Amphipoda (Phronima has o, 

 Caprella 5, Gammarus 7 pairs). 



