i6 4 



HISTOLOGY 



must occur in a degree before the clot can form. The process is not 

 well developed in many animals, to which a cut or other injury becomes 

 a serious matter. In the Crustacea there is a special arrangement con- 

 nected with most of the limbs that greatly aids the clotting. This con- 

 sists of an invaginated ring of the integument, leaving only a small open- 

 ing through which the artery and nerve pass. When the limb is injured, 



and the animal is threatened 

 with a severe bleeding, it has 

 the power to break the limb 

 off at this point so that there 

 is only one small artery and 

 a vein to close, instead of a 

 loose system of lacunae. This 

 apparatus also serves an- 

 other purpose with which we 

 shall not here deal. 



The process of clotting 

 often results in a change of 

 color in the clotted blood. 

 This color is very varied in 

 the lower animals. It is 

 black in most of the insects 

 and many shades of blue or 

 red, especially the dull 

 shades of those colors, in the 

 other invertebrates. 



The blood corpuscle, as 

 has been said, is a cell that 



is free in the blood and has assumed a variety of forms and structures 

 according to the purposes that it has to serve in the different cases. In 

 its simplest form, perhaps, it is a fairly large amoeboid cell with a 

 sharp outline and a good-sized nucleus that may be of very irregular 

 shape or even multiple. The cytoplasm is abundant and filled in 

 most cases with round granules of large size. The number and size of 

 the granules varies in the same kind of cell and is probably dependent 

 upon the state of activity of the cell or upon its age. Notice the dif- 

 ferent conditions of cytoplasmic granulation in the lobster's blood cor- 

 puscles in Figure 146; also see the same feature in the white blood 

 corpuscles of many other animals (see Figs. 139, 143). Figure 147 

 shows one of the white blood corpuscles of the salamander. This 

 shows an excessive development of these structures, and the same 

 can be seen in many other forms. This specimen also shows another 

 organ characteristic of the white corpuscle. This is the centrosome 



FIG. 146. Part of a blood channel of the lobster, 

 containing a very coarsely granular coagulum of 

 lymph body, and blood cells (bl.c.) of several degrees 

 of granule development. X 400. 



