180 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
these is sometimes so great, that the matrix is propor- 
tionally very much reduced, and the structure acquires a 
close superficial similarity to that of the parenchyma of 
plants. This is still more the case with a third form, in 
which the matrix itself is marked off into elongated or 
rounded masses, each of which has a nucleus in its 
interior (fig. 51, B). Under one form or another, the 
connective tissue extends throughout the body, ensheath- 
ing the various organs, and forming the walls of the blood 
sinuses. 
The third form is particularly abundant in the outer 
investment of the heart, the arteries, the alimentary 
canal, and the nervous centres. About the cerebral and 
anterior thoracic ganglia, and on the exterior of the 
heart, it usually contains more or less fatty matter. In 
these regions, many of the nuclei, in fact, are hidden by 
the accumulation round them of granules of various 
sizes, some of which are composed of fat, while others 
consist of a proteinaceous material. These aggregates . . 
of granules are usually spheroidal; and, with the matrix in 
which they are imbedded and the nucleus which they sur- 
round, they are often readily detached when a portion of 
the connective tissue is teased out, and are then known as 
fat cells. From what has been said respecting the dis- 
tribution of the connective tissue, it is obvious that if 
all the other tissues could be removed, this tissue would 
form a continuous whole, and represent a sort of model, 
or cast, of the whole body of the crayfish. 
