STRUCTURE OF CRAYFISH 33 1 



blood is shed, and the plasma surrounding them solidifies 

 in small clots or " islands " ; at a later stage there is a 

 further disintegration of corpuscles and more complete 

 coagulation. Coagulation of the blood is not uncommon 

 in Invertebrates, but is always due to an agglutination, with 

 or without disintegration, of the cells ; only in Crustacea 

 does an actual solidification of the fluid plasma play a 

 part, as in Vertebrates. 



The blood of Crustacea also resembles that of Verte- 

 brates in that it serves as a vehicle for the transport of 

 specific substances, " hormones " or " internal secretions " 

 from one part of the body to another. Some such hormone 

 has been shown to play a part in the adaptive colour change 

 of the shrimp {Crangon). In Asellus the stimulus for the 

 formation of the brood-pouch seems to be an internal 

 secretion of the ovary. The evidence that true hormones 

 occur in other Invertebrates is not yet decisive. 



Respiratory system. — Twenty gills — vascular out- 

 growths of the body wall — lie on each side of the thorax, 

 sheltered by the flaps of the shield. A current of water 

 from behind forwards is kept up by the activity of the baling 

 portion, or scaphognathite, of the second maxilla. Venous 

 blood enters the gills from the ventral sinus, and purifled 

 blood leaves them by the six channels leading to the peri- 

 cardium. 



Observed superficially, the gills look somewhat like 

 feathers with plump barbs, but their structure is much more 

 complex. The most important fact is that they present a 

 large surface to the purifying water, while both the stem 

 and the filaments which spring from it contain an outer 

 canal continuous with the venous sinus, and an inner canal 

 communicating with the channels which lead back to the 

 pericardium and heart. 



Three sets of gills are distinguishable. To the basal joints of the 

 six appendages, from the second maxillipede to the fourth large limb 

 inclusive, the podobranchs are attached. They come oflf with the 

 appendages when these are pulled carefully away, and each of them 

 bears, in addition to the feathery portion, a simple lamina or epipodite. 

 The membranes between the basal joints of the appendages and the 

 body, from the second maxillipede to the fourth large limb inclusive, 

 bear a second set, the arthrohranchs, which have no epipodites. In 

 Qorinection with the second maxillipede there is a single arthrobranch ; 



