582 THE ANATOMY OF INVERTEBRATED ANIMALS. 



The lowest known term of the Arthrozoic series is a Nema- 

 toid worm ; that of the Aimuloid series is a low Turbellarian 

 or Rotifer ; that of the Malacozoic series is an entoproctous 

 Polyzoon ; that of the Pharyngopneustal series is probably 

 most nearly exemplified by the young larva of Balanoglos- 

 sus ; that of the Echinodermal series by the vermiform Echi- 

 nopcedium. 



But the differences between one of the simpler Nematoid 

 worms, an aproctous Turbellarian, a Rotifer, an Echinopsedium, 

 and a Pedicellina, are relatively so small, that all six series 

 may be said to converge toward a common form ; and that 

 common form, when the special characters of each group are 

 eliminated, and the alimentary canal is reduced to its primi- 

 tive aproctous condition, would be exceedingly similar to a 

 Physemarian. 



Hence the consideration of the gradations of structure 

 which are presented by the various series of Invertebrated 

 animals, irresistibly leads to the conclusion that the whole of 

 the Metazoa may be conceived as diverse modifications of a 

 common fundamental plan. 



The Serial Relations or the Invertebrata com- 

 pared with the Results of Embryology. — The conception 

 of the unity of organization of the Invertebrata thus reached, 

 so far as it is based upon the comparison of adult structures, 

 is purely ideal ; and the study of the development of individ- 

 ual animals is alone competent to decide the question whether 

 this ideal unity has a foundation in objective fact. But the 

 history of the development of animals appertaining to every 

 group of the Invertebrata which has been given, bears out 

 the statement which is made in the Introduction, that the 

 ideal unity has such a foundation in fact ; inasmuch as all 

 these animals commence their existence under the same 

 form — that, namely, of a simple protoplasmic body, the ovum 

 or germ. 



In the Introduction I have said that, "among the lowest 

 forms of animal life, the protoplasmic mass which represents 

 the morphological unit may be, as in the lowest plants, devoid 

 of a nucleus " (p. 18). However, as I have remarked at the 

 commencement of this chapter, until the search for the nucleus 

 has been instituted afresh, with the help of such methods as 

 have recently proved its existence in the Foraminifera, I 

 think it will be wise to entertain a doubt whether any of the 

 Monera are really devoid of this amount of structural differ- 



