the Inverteh rata in relation to Evolution. 395 



The Annelida may be linked w-ith the Crustacea by means 

 of the Sagittidse, whose exquisitely striped muscular fibres accord 

 to them a higher position than the other parts of 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 Geopliilus 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 fii'st appearance in the lulidae, 



Should the simplest hydroid polyps have sprung from such 

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

 teutacula encircling a fixed oral point, the existence of a living 

 series from the lowest'type of animals to that which is ob\iously 

 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 miglit be distributed in a similar way into as many groups or 

 series as their afiinities 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 \Aith the number of sections of the 

 Protozoa, from which all the other types may have taken their 

 origin. Thus, on dividing the Astoiiiatous Protozoa into com- 

 pound t\^es 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 exliibits the progressive raodification of the alimentary 

 system in ascending from the Hydrozoa to the Tunicata : — ■ 



Evolution of the Alhnentary Canal in particular. 



( With primary hivmal ] 



T . .. . 1 i ii> I and final neural )■ Ascidiozoa. 

 /Intestine insulated from J n 



MoLLUSCOiDA I ihe somatic cavity . . . ] with'^Trnple ' neural ] Brachiapoda 

 (including ^ y fl^^^^ ^ I ^^^ p^,^,^^^ 



Ltenopliuraj j i^tgg^ino straight, and communicating with 1 (Jf^„^}^^ra 

 \ the somatic cavity J " 



CCELENTERATA 



r Intestine not yet developed ; stomach commu- "1 ^.j,--j_ .. 



I nicating with the somatic cavity J 



I True stomach not yet developed, its office being 1 jTydrozoa 

 l_ answered by the somatic cavity J ^ 



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

 author " On the Morphological Relationships of the Molluscoida and Coclen- 

 terata," published in the Transactions of the Koyal Society of Edinburgh, vol. 

 xxiii. part 3, 1804. 



