the Invertehrata in relation to Evolution. 395 



The Annelida may be linked with the Crustacea by means 

 of the Sagittidse, whose exquisitely striped muscular fibres accord 

 to them a higher position than the other parts o£ their organization 

 would perhaps warrant them to take. 



There is obviously a representative relationship between the 

 crustaceous 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 Chsetopod Annelida and the Myriopoda (as, for 

 example, between the genera Geophilus and Nereis), though it must 

 be confessed that the existing 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 lulida?. 



Should the simplest hydroid poly|Js have sprung from such 

 Protozoa as Diffiugia, Arcella, or Astrorhiza, with their pseudopodial 

 tentacula encircliLg a fixed oral point, the existence of a hving 

 series from the lowest type of animals to that which is obviously 

 on the confines of the Vertebrata would be clearly demonstrable*. 

 Furthermore, as the interpolation 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 antipathies 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 com- 

 pound types and their allied simple forms, we obtain the follow- 

 ing highly suggestive arrangement, in which the groups represent 

 each other so remarkably that they would seem to be quite natural. 



* The annexed Table exhibits the progressive modification of the alimentary 

 system in ascending from the Hydrozoa to the Tunicata : — 



Evohdion of the Alimentary Canal in -particular. 

 ( With primary ha-mal 

 / Intestine insulated from 

 MoLLUscoiDA the somatic cavity ... 1 witirTmple ' neural f Brachiopoda 



(mcluding J \^ ilexure.f. / and iWy-Ofl. 



Ot«nophora) | jj^^gg^ine straight, and communicating with | ctenophora. 

 \ the somatic cavity J " 



(Intestine not yet developed ; stomach commu- \ j^^j^^^^g 

 nicating with the somatic cavity J 

 True stomach not yet developed, its office beingl ^ , ^.^^ 

 answered by tiio somatic cavity J ^ 



Additional matter in the above coimexion will be found in a paper by the 

 author " On the Morphological Eelationships of the Molluscoida and Coelen- 

 terata," published in the Transactions uf tlie Eoyal Society of Edinburgh, vol. 

 xxiii. part 3, 18G4. 



and final neural \ Ascidiozoa 

 flexure . 



