BLOOD AND ITS CIRCULATION 45 



Another function of the blood cells is to plug small wounds. The 

 cells gather around the wound and form a mass in which all the 

 cell bodies unite. This mass serves as a stopgap until the tissues 

 around the wound can regenerate and repair the damage. 



Some of the blood cells are concerned with the transport of 

 materials around the body. This is particularly well shown in the 

 shore crab when it is about to moult. Large blood cells are found, 

 filled with a lipoprotein substance, which is similar to that forming 

 the outer layer of the cuticle. When the crab moults these cells 

 disappear. These lipoprotein cells appear to transport the material 

 to the bases of the epidermal cells so that they can use it to manu- 

 facture the outer layer of the exoskeleton. It is not certain where 

 the blood cells get the lipoprotein from, or whether they make it 

 themselves. 



The mechanism for circulating the blood of Crustacea is variable. 

 Some crustaceans (e.g. harpacticid copepods) do not have a heart. 

 This seems to be connected with their small size; the blood moves 

 sufficiently when the animal moves its limbs. 



When a heart is present it lies in the dorsal part of the body, 

 just beneath the exoskeleton. Fig. 26 gives some idea of the range 

 of shape and position of the heart and main arteries. The cavity 

 containing the heart is separated from the rest of the body cavity 

 by a membrane, the pericardium. Blood enters the heart from the 

 pericardial cavity. Small valves, or ostia, allow the entry, but not 

 the exit of blood. When the heart contracts the blood is forced out 

 through other apertures, which may, or may not be continued into 

 an arterial system. In Daphnia there is a single exit at the front of 

 the heart, which is slightly drawn out to form a very short ' aorta '. 

 In contrast, the Decapoda and Isopoda have an elaborate system 

 of arteries leaving the heart in various directions. The venous system 

 is represented by ill denned sinuses. Generally, there is a large 

 sternal sinus, below the gut; this sinus communicates with the 

 cavities of the gills and the limbs. A series of canals runs from 

 the gills upwards to the pericardial cavity. In some species, such as 

 the crayfish, these canals are narrow and well defined enough to be 

 called branchio-cardiac veins. 



When the heart contracts its volume naturally diminishes, and 

 pressure within the pericardium is lowered; this results in blood 

 being drawn into the pericardial cavity. The filling of the crusta- 

 cean heart is a very different process from that in mammals, where 

 the blood circulates in closed vessels throughout its course and so 



