THE CIRCULATORY SYSTEM 33 



over, they have a habit of spreading themselves out in stellate form 

 upon flat surfaces, such as the basement membranes, their aspect 

 becoming totally changed. 



These haemocytes are by no means all circulating; indeed, the 

 majority adhere to the surface of the tissues; they collect particularly 

 along the sides of the dorsal vessel, often in definite clumps termed 

 'phagocytic organs'. The larvae of Chironomidae present a complete 

 series in this respect: in certain genera only free haemocytes are 

 present; in others, both haemocytes and fixed phagocytic cells occur; 

 and in yet others there are no circulating cells at all but only phago- 

 cytic tissue. The haemocytes become far more numerous during 

 moulting and metamorphosis, when they may play a part in removing 

 the dead cells and tissues: during the metamorphosis of Muscids, 

 these phagocytic cells (the 'Kornchenkugeln' of Weismann), stuffed 

 with the products of histolysis, are exceedingly conspicuous. The 

 phagocytic haemocytes will ingest, and in some cases destroy, living 

 bacteria introduced into the blood. They collect at the site of wounds 

 and form a closing plug which forms the basis for the subsequent 

 healing process. They will congregate in great numbers around 

 foreign bodies and certain parasites, walling these off in a discrete 

 capsule; sometimes the celullar nature of this capsule persists, but 

 more often the cell bodies become converted into homogeneous 

 membranes from which the nuclei disappear. The plasmatocytes 

 often contain abundant inclusions of neutral mucopolysaccharide 

 which are discharged when the cells apply themselves to the surface 

 of the tissues during moulting. They probably contribute to the 

 connective tissue membranes. In some insects, haemocytes laden with 

 conspicuous granules (granular leucocytes) are present in the blood; 

 although these are no longer phagocytic they seem to represent a 

 stage in the life history of these same cells. 



Besides this type of cell with all its varied powers, there is, in most 

 insects, another type which is equally distinct. It has a rounded or 

 oval form, the cytoplasm is generally eosinophil, and it takes no part 

 in phagocytosis nor in the formation of encapsulating membranes. 

 In appearance these cells are like diminutive oenocytes; and on this 

 account they are often called 'oenocytoids' ; but they do not seem 

 to bear any relation to the true oenocytes. Their function is quite 



