ARTHROPODA 279 



in the Onychophora, the Arachnida, and the Insecta and Myriapoda. 

 Among the Crustacea, tufts of tracheae are found in the abdominal 

 appendages of woodlice. 



The vascular system is an "open" one. That is, be the arteries long 

 or short, they end by discharging their blood not into capillaries in 

 the tissues from which veins conduct it to the heart, but into peri- 

 visceral cavities, known as sinuses ^ which bathe various organs. 

 From these sinuses the blood collects into a pericardial sinus 

 ("pericardium"), part of the haemocoelic system, which surrounds 

 the heart. The latter is a longitudinal dorsal vessel, perforated by 

 ostia by which it receives its blood from the pericardial sinus. Among 

 the consequences of the structure of the vascular system are a low 

 blood pressure and liability to severe bleeding from wounds. The 

 latter danger is met, especially in the Crustacea, by very rapid 

 clotting of the blood. 



The coelom appears in the embryo as the cavities of a series of 

 mesoderm segments (" mesoblastic somites", Fig. 328). It never 



Fig. 202. Three stages in the cleavage of the egg of Astacus. After Morin and 

 Reichenbach. nu, nuclei; yp, "yolk pyramids", due to the transitory appear- 

 ance of divisions of the yolk corresponding to the superficial cells. 



assumes a perivisceral function, and in the adult is always reduced 

 to the cavities of the gonads and of certain excretory organs. 



The excretory organs of arthropods are of very various kinds. True 

 nephridia appear never to be present. Coelomoducts are present in a 

 number of cases, though in the absence of perivisceral coelom they 

 end internally each in a small coelomic vesicle or "end sac". These 

 are found in the Onychophora in a long series of segmental pairs. In 

 Crustacea there is either a pair of coelomoducts on the third (antennal) 

 somite or a pair on the somite of the maxillae, or, rarely, both these 

 pairs are present. In various crustaceans other glands, some ecto- 

 dermal, some mesodermal, appear to have an excretory function, and 

 sometimes replace both pairsof coelomoducts, which become vestigial. 

 In arachnids, coelomoducts open on one or two of the pairs of legs. 

 They are known as coxal glands^ but are not homologous with the 

 glands to which that name is applied in certain crustaceans. Mai- 

 pighian tubules are tubular glands which open into the alimentary 



