THE DICYEMIDiE. 



69 



the endodermal cell in the axis of the body becomes an organ of 

 nutrition, and takes on the function of repro- 

 duction, for the germs of young forms are found 

 in it of two different types. 



The organism of Dicyema is thus seen to be 

 two-layered, and the two layers have different 

 functions ; the inner one is morphologically least 

 differentiated, for it consists of a single cell. 

 It is not quite certain whether this corresponds 

 to a primitive condition or no, for the parasitic 

 life of the Dicyemidas may have been the cause 

 of the reduction of a multicellular endoderm. 

 But their development is quite easy to make 

 out, and in it there is never more than one 

 endodermal cell, so that the fact becomes more 

 significant. Just as among the Protozoa the Ms 



multicellular forms are allied by intermediate 

 steps to the unicellular, from which they have 

 been developed, so among the Metazoa does 

 Dicyema exhibit to us the commencement of the 

 separation of the body into cell-layers, and 

 although the arrangement is not as perfect as it 

 is in the rest, yet it is in the same direction as 

 that which in them arrives at full expression. 



Vax Beneden, Ed., Recherches but les Dicyemida?. Bull. 

 Acad. Belg. xli., xlii., 1876. 



§ 58. 



Passing" over the Dicyemida3, I recognise the 

 following divisions of the Metazoa : 



1. Ccelenterata. 



2. Vermes. 



3. Echinoderma. 



4. Arthropoda. 



5. Brachiopoda. 



6. Mollusca. 



7. Tunicata. 



8. Vertebrata. 



Fig. 21. Vermiform 

 embryo of Dicyema 

 typus. n Nucleus of 

 the endodermal cell 

 (after E. van Bene- 

 den). 



These divisions represent in a general way separate branches of 

 the pedigree of animals, and each of them contains higher and lower 

 forms in various proportion. But the degree and extent to which 

 their organisation is developed is different in each of them. The 

 divergence of organisation expressed in each division is indicated by 

 their relations to one another, and it shows us how the lower forms 

 of the higher phyla may have started from the lower phyla. These 



