No. 3.] ZHE EMBRYOLOGY OF THE EARTHWORM. 429 
be taken to indicate with considerable probability the primitive 
character of such a feature, and that, conversely, those features 
in which the two types differ are due to secondary modification 
having taken place in one or both forms. Obvious as this prin- 
ciple appears, its importance seems to me not to have been 
sufficiently recognized in a number of recent papers on the sub- 
ject, and it must carefully be kept in view in the following dis- 
cussion. 
XI. RELATIONS oF THE HEAD! (Prostomium) AND TRUNK. 
The most fundamental question of annelidan morphology con- 
cerns the relation between the head and the trunk. Indeed, it 
is not too much to say that this question, involving as it does 
the interpretation of the Trochosphere, of the teloblasts, and of 
metameric segmentation, is one of the most important problems 
of comparative morphology, though it is admitted by a number 
of leading morphologists to be incapable at present of more 
than a conjectural solution. The limits of this paper do not 
permit a review of the very extensive literature on the subject, 
and I shall therefore refer at this point to only two of the latest 
papers. Fraipont has given in his monograph of Polygordius 
(No. 15) an admirable review of recent discussions of the Tro- 
chosphere, to which Whitman (No. 52) has added an important 
discussion of the teloblasts and of growth by concrescence, — 
questions which were not specially considered by Fraipont. 
Both these authors admit that the Trochosphere larva of anne- 
lids still remains (to use Whitman’s phrase) a morphological 
puzzle, which can only be solved by further extended investiga- 
tion —an admission which is fully justified by the diametrical 
opposition of the views of leading authorities on the subject. 
It would therefore be profitless to enter upon an exhaustive dis- 
cussion, and I wish only to make a few suggestions in regard to 
certain points which do not seem to me to have been sufficiently 
considered and on which a certain amount of light is thrown by 
the development of Lambricus. They must, however, be intro- 
duced by a short review of the general question. 
1] shall use the word “ head” to designate that part of the body (often called pro- 
stomium, prz-oral lobe, or cephalic lobe) that contains the anterior median unpaired 
cavity that lies in front of the first dissepiment, and obviously represents the principal 
cavity of the Trochosphere. 
