i.] GENERAL VIEW. I3 



communication takes place between the interior of the circu- 

 lating system and the exterior of the body, so that the water 

 in which such creatures live is admitted within their blood 

 system to a greater or less extent. 



1 8. A distinct brain is common to all the members of man's 

 sub-kingdom except the lancelet ; but both nerves and ner- 

 vous centres may be entirely wanting in creatures of complex 

 structure like the tapeworm, or of locomotive power like the 

 polyp, Hydra tuba, 



Respiration of air by pulmonary sacs is neither universal 

 in man's sub-kingdom (for fishes breathe the air contained in 

 water by gills), nor unknown out of it. Such structures exist 

 in scorpions and spiders, and an air-breathing sac is found 

 in slugs and snails. 



On the other hand, breathing organs suited for aquatic 

 respiration may be developed in the most varied situations ; 

 e.g., attached to the legs, as in the lobster, or internal with 

 an external opening at the hinder end of the body, as in the 

 sea-cucumber, Holothuria. 



Kidneys, instead of being distinct structures as in man, 

 may be united with biliary glands and open into the intes- 

 tine, as in insects. 



19. Sense organs may exist in quite other situations than 

 those in which they are placed in man. Eyes may be placed 

 on every joint of a long body, as in the worm-like animai, 

 Polyophthalwus* or along each half of a fleshy cloak enclos- 

 ing the body, as in Pecten. There may often be more than 

 two upon the head, as in Scorpio; or they may be raised on 

 solid stalks, as in the lobster ; or at the end of long retractile 

 tentacles, as in the snail. 



Auditory organs may be placed in joints of the legs, as 

 in some insects, e.g. locusts ; or in the thoracic part of the 

 body, as in crickets. 



Long filamentary jointed structures (the antennas of insects) 

 may project from the head, to minister to a sense the nature 

 of which is disputable. 



The mouth, instead of being extended transversely, may be 

 rather antero-posteriorly elongated even in man's sub-king- 

 dom, as in the lancelet ; and there may be numerous jaws 

 placed on each side of it working laterally, and not vertically, 

 as in the lobster. 



20. Development may take place without any primitive 

 groove and without any chorda dorsalis, and such is the case 



1 From -oAuc, many'; 6'<ji0aX/uo, an ev<*. 



