222 



Dr. J. D. Macdonald on the Distribution of [Mar. 20, 



There is obviously a representative relationship between the crusta- 

 ceous Macrura, Anomura, and Brachyura and the chitinous Myriopoda, 

 Insecta, and Arachnida. 



The earthworms and the leeches may help to fill up the gap between 

 the Cha>topod Annelida and the Myriopoda (as, for example, between the 

 genera Geojihilus and Nereis), though it must be confessed that the exist- 

 ing links are inadequate, or they have never been sufficiently made out. 



The first rudiments of a tracheal system are probably to be sought for 

 in the Terricolous Annelida, though true articulated limbs and a dorsal 

 heart seem to make their first appearance in the Iulidae. 



Should the simplest hydroid polyps have sprung from such Protozoa 

 as Difflugia, Arcella, or Astrorliiza, with their pseudopodial tentacula 

 encircling a fixed oral point, the existence of a living series from the 

 lovest type of animals to that which is obviously on the confines of the 

 Vertebrata would be clearly demonstrable*. Furthermore, as the inter- 

 polation of any other invertebrate types would disturb the harmony 

 here, the inference is natural that they also might be distributed in a 

 similar way into as many groups or series as their affinities or anti- 

 pathies would suggest or necessitate. 



Having studied this subject very carefully, it appears to me that the 

 whole of the Invertebrata admit of distribution into four distinct series, 

 corresponding with the number of sections of the Protozoa, from which 

 all the other types may have taken their origin. Thus, on dividing the 

 Astomatous Protozoa into compound types and their allied simple forms, 

 we obtain the following highly suggestive arrangement, in which the 

 groups represent each other so remarkably that they would seem to be 

 quite natural. I have appended the Stomatoda and the twelve remaining 

 sections of the Invertebrata in the order indicated by their affinities. 



* The annexed Table exhibits the progressive modification of the alimentary system 

 m ascending from the Hydrozoa to the Tunicata 



Additional matter in the above connexion will be found in a paper by the author 

 ' On the Morphological Relation ships of the Molluscoida and Ccelenterata," published, 

 in the Transactions of the Royal Society of Edinburgh, vol. xxiii. part 3, 1864. 



Evolution of the Alimentary Canal in particular. 



la and 



