1 2 ELEMENTAR Y ANA TOM Y. [LESS. 



descend in the animal scale, but a liver is a structure which 

 long persists, as e.g. in the Mollusca and many Annulosa. 



1 6. In that man's body consists of a double tube the 

 nervous cylinder being dorsal and the heart ventral it agrees 

 with the whole of the members of his sub-kingdom, but 

 appears to differ from that of all invertebrates, unless it be 

 certain Ascidians. Recent observations show that the latter 

 simulate in their larval condition the vertebrate structure as 

 regards the existence of a dorsal nervous cylinder, as also as 

 regards a solid partition or simulation of a notochord, which 

 nevertheless is not yet unequivocally manifested, outside the 

 Vertebrate sub-kingdom. 



A prolongation of the body cavity and even of the digestive 

 cavity into the limbs, strange as it may appear, is far from 

 unknown, as it exists not only in the star-fishes, but also in 

 animals as highly organized as spiders. 



A direct communication between the alimentary canal and 

 the true body-cavity is by no means uncommon in lowly orga- 

 nized forms ; e.g., in the sea-anemone that canal terminates 

 freely in the body-cavity. 



In creatures in which the central part of the nervous system 

 is ventral that is to say, in all the Annulosa the anterior 

 part of the alimentary tube not only bends towards but 

 traverses the nervous centres. That bending away from 

 those centres which characterises it in man, characterises it 

 also in all vertebrate animals. 



Speaking generally, the nervous centres of invertebrates are 

 placed in the oral region, as not only in the Annulosa and 

 Annuloida, but also in the Mollusca and Molluscoida, it is 

 present in that part, unless absent altogether. 



17. The possession of a heart and of red blood is common to 

 all vertebrates as well as to man, with 

 one solitary exception : the Amphioxus 

 or lancelet alone having colourless 

 FIG. 13. THE LANCELET blood, and a simple cylindrical vessel 



A portal circulation is shared with 



man by all vertebrates, even the Amphioxus, but no inverte- 

 brate animal is known to be furnished with a blood distribu- 

 tion of the kind. 



Also the enclosure of the blood in vessels is a cha- 

 racter common to vertebrates, but in Annulose animals (e.g. 

 the lobster) the blood is in part contained in wide cavities 

 termed sinuses. In many forms also (as in the Mollusca) a 



