430 WILSON. [Vot. III. 
Either the Trochosphere larva of annelids is the larval repre- 
sentative of an ancestral non-metameric form, as held by Hat- 
schek, Kleinenberg, and Whitman, or it is a purely secondary 
form as held by Lang and Sedgwick. Under the former view the 
head must be regarded as phylogenetically the oldest part of 
the animal, and the trunk as a neomorph, the origin of which 
involved the origin of metamerism. According to the second 
view, head and trunk are but secondarily differentiated parts of 
the ancestral body, and the retardation or rudimentary condition 
of the metameric trunk-region that now characterizes the Trocho- 
sphere is due to cenogenetic changes ; thus the origin of meta- 
merism may have been antecedent to the appearance of the 
Trochosphere, and the head not phylogenetically older than 
the trunk. 
There is no means of deciding between these two conflicting 
views save by a thorough investigation of the actual anatomical 
and embryological relations between the head and trunk—a 
question which I shall now briefly review. 
The head has been asserted to differ morphologically from 
the trunk in the following principal characters : (1) in containing 
no reproductive organs ; (2) in arising from a pair of mesoblastic 
foundations that arise independently of the trunk mesoblast ; 
(3) in containing an unpaired cavity (“primary body-cavity”’) 
which is the remains of the blastoccel traversed by mesenchy- 
matous cells, and not homodynamous with the coelomic cavities 
of the trunk (“secondary body-cavity”’), since these arise by 
cleavage of the mesoblast and are surrounded by mesothelial 
walls; (4) in possessing a nervous system (Scheitelplatte, of 
Hatschek, or apical neural plate) which is primitively unpaired 
(Hatschek), and arises independently of the ventral chain (Klein- 
enberg, Salensky, etc.). 
Let us examine these statements. The first may be set 
aside, since many of the trunk somites are sterile like the head. 
The second has been disproved so far as the Che@topods are 
concerned, by Hatschek, who has clearly shown that the head- 
mesoblast arises by the forward growth and union of the two 
mesoblastic trunk-bands, a conclusion which Kleinenberg like- 
wise reached in his later studies (Lopadorhynchus), and with 
which my own observations entirely agree. 
The third statement is true as far as the unpaired character 
