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SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 97 



in fact, the later horizontal development of the worm form along 

 the vertical axis of the larva shows clearly that the trochophoral 

 position is one quite out of harmony with the general organization 

 of a worm. 



The trochophore, therefore, is to be regarded as a temporary, 

 specialized larval form in polychaete ontogeny, adapted to a free 

 pelagic life for the purpose of disseminating the individuals of its 

 species. The metamorphic alterations that it undergoes at its trans- 

 formation to the worm are changes of a nature that could not have 

 been a part of the phylogenetic evolution of any animal. On the 



_ Fig. 4. — The polychaete trochophore and the crustacean nauplius, two spe- 

 cialized larval forms of an early ontogenetic stage, having, therefore, primi- 

 tive characters, but no phylogenetic significance in their shape or general 

 structure. 



A, typical structure of a trochophore, diagrammatic. B, nauplius of a cirriped, 

 Alcippe lampas Hancock, dorsal surface (from Kiihnert, 1935). 



An, anus ; lAnt, first antenna ; 2Ant, second antenna ; ApGng, apical ganglion ; 

 ApPl, apical plate; CNv, circular nerve; Epsp, episphere; Hpsp, hyposphere; 

 Ih, lateral horn; LNv, longitudinal (radial) nerve; Md, mandible; Mcnt, 

 mesenteron; MsT, mesodermal teloblast; Mth, mouth; Nph, nephridium; O, 

 naupliar ocellus; Pair, paratroch; Proc, proctodaeum; Prtr, prototroch; Stom, 

 stomodaeum. 



Other hand, inasmuch as the trochophore is an early ontogenetic stage, 

 its general organization is primitive, and is repeated in the onto- 

 genetic development of many other invertebrates besides the annelids. 

 It should be noted, furthermore, that the trochopore is not a uni- 

 versal larval form even among the annelids, for most of the archi- 

 annelids, some of the polychaetes, and all the oligochaetes have a 

 direct development, in which either there is no suggestion of the 

 trochophore form, or a remnant of it is preserved from ancestors 

 that had a typical swimming larva. 



