138 HISTORICAL REVIEW. 



the absolute independence and isolation of each group must be given 

 up. With a more complete study it has been shown that inter- 

 mediate forms exist connecting the various types, and that conse- 

 quently no sharp line of demarcation can be drawn between them. 

 But just as the transitional forms between animals and plants cannot 

 abolish the distinction between these two most important conceptions 

 of the organic kingdom, so the existence of such transitional forms 

 does not in any way affect the value of the idea of groups and types 

 as the chief divisions of the animal system, but only renders it 

 probable that the different groups have developed from a similar or 

 common starting-point. 



And to this corresponds the fact, which has become recognised 

 with the progress of embryological knowledge, that similar larval 

 stages and tissue-layers (germinal layers) are found in the develop- 

 mental history of the different types. This fact points to a genetic 

 connection. 



Likewise the results of anatomical and embryological comparison 

 have rendered it probable that the types are by no means absolutely 

 independent, but are subordinated to one another in more or less 

 close relation, that especially the higher groups are genetically to be 

 derived from the Worms, a group which certainly includes extremely 

 dissimilar forms, and eventually will, without doubt, be broken up 

 into several types. We consider it, under such circumstances, con- 

 venient, in the present state of science, to distinguish nine types as 

 the chief divisions, and to characterise them in the following 

 manner : 



(1) Protozoa. Of small size, with differentiations within the sar- 

 code, without cellular organs, with predominating asexual repro- 

 duction. 



(2) Ccelvnterata. Radiate animals segmented in terms of 2, 4, or 

 6 ; mesoderm of connective tissue, often gelatinous ; and a central 

 body cavity common to digestion and circulation (gastro-vascular 

 space). 



(3) Echinoderrnata. Radiating animals, for the most part of pen- 

 tamerous arrangement ; with calcareous dermal skeleton, often bear- 

 ing spines ; with separate alimentary and vascular systems ; and with 

 nervous system and ambulacral feet. 



(4) Vermes. Bilateral animals with unsegmented or uniformly 

 (honionomous) segmented body, without jointed appendages (limbs), 

 with paired excretory canals sometimes called water- vascular system. 



(5) Arthropoda. Bilateral animals with heteronomously segmented 



