CR USTACEA— BLOOD-VASCULAR SYHTEM 



367 



with it. The respiratory organs are branched tufts which arise on the inner surface 

 of the branchiostegite. The shell circulation which is found in all Thoracostraca 

 and which also plays a great part in the respiration of many water -breathing 

 forms, here brings about air-breathing. In the branchiostegite (here better 

 "lung cover") and its tuft-like appendages there is a rich meshwork of blood 

 smuses, spread out between the vessels which conduct the blood in and out. 

 The blood passes through a large vessel out of the venous blood sinus of the 



Fig. 245.— Birgus latro. Diagrammatic transverse section in the region of the heart (after 

 Semper). W, GiU or lung cover ; 7i, heart ; fc, gills ; ah, respiratory cavity ; p, pericardium ; ek, 

 branchial blood channels leading to the heart ; cii, a^, 03, 04, lung or shell vessels leading from the 

 heart ; U, pulmonary tufts ; d, pulmonary blood channels leading to the heart ; eli, the same near 

 their entrance into the pericardium. 



head into the lung cover. This vessel divides into 4 branches, 3 of which run 

 to the upper and 1 to the lower portion of the lung cover, and break up into 

 the meshwork of blood sinuses above mentioned. From this the blood which 

 has become arterial is collected by vessels which unite into a great trunk, running 

 first along the edge of the lung cover backwards, then upwards, and finally forwards, 

 to unite before entering the pericardium with the vessel coming from the small gills. 



The blood of the Crustacea is usually colourless ; it is occasionally, 

 ho^vever, slightly yellow, green, or red. In the latter case, e.g. in the 

 Branchiopoda, the colouring material of the blood is hsemog'lobin. 

 The colourless blood corpuscles are almost always able to change their 

 form in an amseboid manner. 



Judging from the varying position and form of the Crustacean 

 heart, we come to the conclusion that the original Crustacean heart 

 was, as in Branchijous, a long, many- chambered dorsal vessel pro- 

 vided with many segmental pairs of ostia. All other forms of 

 heart have been developed from this by reduction of the anterior 

 or the posterior portion, and by the disappearance of numerous 

 pairs of ostia. These reductions have had for their principal causes 

 the localisation of the respiration, the various diiferentiations of the 

 different portions of the body, and the fusing of segments. The 

 already mentioned ontogenetic fact that pairs of ostia may disappear 

 in the course of development {Apseudes, Stomatopoda), agrees -mth this 

 view. In many Isopoda the ostia lie alternately to the right and left 

 in the heart ; this arrangement perhaps comes into existence by the 

 disappearance of alternate ostia in the heart at first provided with 

 paired ostia. 



