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 gi'anules 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. 



