VI ARTHROPODA 235 



body and shedding it periodically, instead of passing it off in solution 

 by means of its nephridial tubes. It is probable that the development 

 of this habit in the Arthropoda has had much to do with bringing about 

 the reduction of the primitive nephridial tubes. 



In the blood-system of the Arthropoda the most important feature 

 is that already alluded to, namely that what would normally be the 

 finer blood-vessels have become greatly dilated, forming a system of 

 irregular spaces which constitute the haemocoelic body-cavity. The 

 blood is driven through these spaces by the beating of the heart, which 

 is here a development of the dorsal blood-vessel — extending it may be 

 through a great part of the entire length of the body or being on the 

 other hand shortened down into a compact organ. 



Whatever be its shape the heart of the Arthropoda always shows this 

 striking peculiarity that its walls are perforated by slits or ostla by which 

 its cavity is in free communication with the body-cavity round it. The 

 explanation of this peculiar circumstance lies of course in the fact that 

 the body-cavity consists really of blood-spaces instead of being coelomic 

 in its nature — and therefore isolated from the blood-system — as it is in 

 most types of animal. 



The blood is as a rule practically colourless but sometimes it is of a 

 bluish colour owing to its containing in solution a bluish copper-containing 

 substance Haemocyanin, possessing the same affinity for oxygen as 

 Haemoglobin has (p. 141) and serving like the latter as a " vehicle " 

 for oxygen. 



The nervous system of the arthropod is constructed on the same 

 general plan as that of the annelid. It has, however, reached a higher 

 stage of evolution : the ganglion cells show a higher degree of con- 

 centration, the ganglia of the ventral cord being more distinct and in 

 particular the supra-oesophageal ganglia or brain having reached a 

 higher level of size and complexity of structure. This latter advance is 

 correlated with (i) the great development of sense-organs in the head 

 region ; (2) the crowding together of the appendages and their associated 

 nerve-centres around and in front of the mouth ; and (3) the general 

 tendency, apparent in the evolution of groups of animals which move 

 actively with one definite end in front, for the control of the activities 

 of the body as a whole to become more and more concentrated in the 

 front portion of the central nervous system. 



The fact that the general surface of the body consists not of soft 

 living protoplasm but of hard chitin involves necessarily peculiarities in 

 the sensory apparatus by which impressions are received from the outer 

 world. Distributed more or less generally over the surface are sensory 



