440 WILSON. [Vou. III. 
centres, precisely analogous to the apical cells of plants, and 
like them adapted to facilitate a rapid and continuous elongation 
of the body in one direction. Morphologically they must be 
taken to represent parts that have been retarded in development, 
and at the same time extremely reduced and concentrated by a 
precocious segregation of material. In other words, the telo- 
blasts represent the rudimentary trunk of the Trochosphere, and 
indicate the former presence in the larva of a developed trunk, 
which is now temporarily reduced in favor of the head, the latter 
having meanwhile acquired special larval organs of locomotion 
and sensation. The evidence at our command appears to me 
to indicate that the annelid Trochosphere is a secondary larval 
form analogous in its mode of origin to the Crustacean Nauplius, 
which was itself so long regarded as an ancestral form. It is 
now generally admitted that the former conception of the Nau- 
plius is no longer tenable,! and that the characteristic features 
of the Nauplius are of purely secondary origin, a few anterior 
somites having been accelerated and specially differentiated to 
meet the requirements of larval existence, while the others have 
been retarded or for the time being entirely suppressed. I can 
see no valid reason against regarding the Trochosphere as hav- 
ing arisen by an analogous process from an elongated segmented 
ancestral form, the head-region or prostomium being enormously 
developed and provided with special organs of sense and loco- 
motion, and the trunk-region more or less retarded, becoming 
reduced, it may be, to a mere trunk-bud, as in the typical larval 
forms. It is no doubt an astonishing fact that the entire meso- 
blastic trunk-region of an animal should have been compressed 
into a single pair of cells, but it is scarcely more astonishing 
than the complete secondary suppression of the long posterior 
metameric trunk-region in the Nauplius. 
It is instructive to notice that, as regards the retardation of 
the trunk-region, a series exists among the annelids, which is, 
broadly speaking, analogous to the series occurring among Crus- 
tacean larve. Lumbricus stands midway between Polygordius 
or Lupomatus on the one hand, and Euwaxes or Clepsine on the 
other. In the latter the mesoblastic bands are fully established, 
and join in front long before the epibolic gastrulation is com- 
1 For reviews of this question, see Claus, No. 13, and Dohrn, No. 14. 
