THE WORMS AS ANCESTORS OF MAN. 75 



not related to the Plant-animals. It will be our next task 

 to consider more closely the pedigree of Man in so far as 

 it coincides with that of the Worms. Let us examine how 

 far the Comparative Anatomy and Ontogeny of Worms 

 justify us in looking among the latter for primaeval ancestors 

 of Vertebrates, and therefore of Man. For this end we must 

 first consider the zoological system of Worms. In accord- 

 ance with the most recent investigations of the Comparative 

 Anatomy and Ontogeny of Worms, we divide (without 

 reference to the many and various peculiarities of the 

 numerous separate classes, which in this place do not 

 interest us) the whole mass of forms within this tribe 

 into two large main groups. The first main group, which 

 we call Bloodless Worms (Acoelomi), comprehends the 

 earlier division of the lower Worms, which have no true 

 body-cavity, no system of blood-vessels, no heart, no blood, 

 — in short, none of the parts connected with this organ- 

 system. The second main group, on the contrary, called 

 Blood- worms {Ccelomati), are distinguished from the former 

 by the possession of a true body-cavity, and also by the 

 presence of a blood-like fluid, which fills this cavity ; 

 most of them also develop special blood-vessels, which 

 again cause further correlated advances in structure. The 

 relation of these two main groups of Worms is very evi- 

 dently phylogenetic. The Acoelomi, which are very nearly 

 allied to the Gastrsea and the Plant-animals, are to be 

 regarded as an earlier and lower group, from which the 

 more recent and higher division of the Ccelomati developed, 

 perhaps towards the end of the Laurentian Period. 



We will first carefully examine the lower group of 

 Worms, the Acoelomi, among which we must look for the 



