202 ARTHROPODA [CH. 



manufacture bundles of white tough fibres which interlace in such 

 a way as to produce a membrane of great strength. A special 

 development of this connective tissue underlies the ectoderm and 

 is termed dermis it is included along with the ectoderm in the 

 popular term skin. But connective tissue is ubiquitous in the 

 crayfish, if as Huxley said we could imagine the essential cells of 

 every organ dissolved there would remain a cast of the whole in 

 connective tissue. 



Blood may be regarded as a tissue of the same kind as connective 

 tissue only here the ground-substance is thoro uglily fluid, the 

 amoebocytes or blood cells not depositing fibres. As already 

 mentioned, in the crayfish all the crevices between the various 

 organs are occupied by blood, these cavities taken together being 

 termed thehaemocoele. Of these the largest is the pericardium, 

 a space just under the dorsal wall of the thorax which is divided 

 from the rest by the horizontal pericardial septum a structure 

 which from observations on the development of other Arthropoda we 

 learn to be derived from the dorsal portions of the coelomic sacs, 

 which have flattened out and lost their cavities. The pericardium 

 communicates with certain vertical canals in the thin side-walls of the 

 thorax (i.e. the true walls, not the branchiostegite), termed branchio- 

 cardiac canals. These canals are in communication with the gills, 

 and through them the blood that has absorbed oxygen there is re- 

 turned to the pericardium. Each gill has a hollow stem traversed 

 by a longitudinal septum which divides its cavity into two spaces, 

 an upper and a lower, only communicating at the tip. The branchio- 

 cardiac canal communicates solely with the upper passage, whilst 

 the lower communicates with the ventral sinus already mentioned, 

 in which the nerve-cord lies, from which the impure blood passes 

 to the gills. 



Returning to the pericardium we find suspended in it by 

 fibrous cords, termed the alae cordis, the heart. This is an 

 oval muscular sac with three pairs of openings, termed ostia 

 (Lat. ostium, a door). Each ostium is provided with two flaps 

 which open inwards, but which when the heart is full of blood are 

 pressed .together so that they meet and prevent the blood from 

 escaping. Thus the heart receives the blood from the pericardium. 

 When the heart contracts the blood is driven out on all sides 

 through vessels with strong muscular walls called arteries. There 

 are six main arteries one called the abdominal goes backwards 

 over the intestine, and gives off a branch immediately behind the 



