ARTHROPODA — CRUSTACEA 281 



anus, which is situated on the ventral surface of the telson. 

 With the exception of the sharp downward bending to the 

 mouth the digestive canal is a straight tube extending the length 

 of the body. 



Since the oesophagus and stomach as well as the large intestine 

 are formed by the inbending of the surface of the gastrula, 

 they are formed from the ectoderm as is the surface layer (epider- 

 mis) of the mature animals ; hence they similarly secrete a Hning 

 of chitin which must be shed with each molt. Only the small 

 intestine and its digestive glands are formed from the inner 

 cell layer, the endoderm, of the gastrula stage. 



The absorption of the digested food takes place through the 

 walls of the small intestine and its digestive glands directly 

 into the blood (since the intestine is surrounded by a blood 

 sinus) and thus finally into the heart. 



The blood circulatory system consists of a muscular heart in 

 the dorsal part of the thorax, arteries, capillaries, blood sinuses 

 (cavities without definite walls among the muscles and viscera) 

 and veins. 



The rhythmical contraction of the heart forces the blood into 

 seven arteries, which, dividing, extend into all parts of the body 

 and finally into microscopic capillaries which terminate by open 

 mouths in the blood sinuses. All blood sinuses communicate 

 with the ventral sinus which runs the entire length of the 

 abdomen and thorax, and conducts the blood to the veins on the 

 gills. From the gills, veins carry it to the sac-like cavity 

 (pericardial sinus) in which the heart is located. The blood 

 enters the heart, upon the latter's rhythmical expansion, through 

 three pairs of apertures wdth inwardly opening valves. The 

 blood is a colorless fluid and all the corpuscles are similarly 

 colorless {i.e. leucocytes.) When combined with oxygen it is 

 bluish gray owing to the haemocyanin in the fluid (not in the 

 corpuscles), which like haemoglobin in mammals has great 

 affinity for oxygen ; it thus acts as the oxygen carrier from the 

 gills to the tissues. 



Respiration is performed by the gills, aided probably by the 

 inner surface of the gill covers which bound them externally. 



