Nows-] “2HE ZMBRYOLOGY OF THE BARTHWORM. 439 
crescence of its lips outstrips that of the other elements of the 
germ-bands, and the anus is retarded in development. Con- 
crescence of the neural and mesoblastic elements, however, 
still follows the original mode of development, the lateral parts 
of both rings long remaining separate behind the point where 
the germ-bands diverge. The long continuance of this diver- 
gence of the germ-bands behind, which is a remarkable feature 
in the development of many if not all annelids, seems to be due 
to the retention in this region of the embryonic condition to 
allow the continued growth and elongation of the body. 
The most obvious objection to this view lies in the fact (first pointed out 
by Hatschek, as far as I am aware) that the proctodzum arises outside the meso- 
blastic ring (¢.e. apparently behind the primary mesoblasts) and, therefore, 
not in the region of the blastopore. This objection is, however, by no means 
a fatal one, either in respect to the relation between anus and blastopore or 
to that between the anus and the mesoblastic ring. The first point has been 
fully considered by Sedgwick in his general discussion, and the differences 
between the foetal and larval types in this respect show that the development 
of the anus has been greatly modified. The second point loses much of its 
force from the facts, first, that the position of the primary mesoblasts is vari- 
able (compare Crzodrzlus, in which the mesoblastic ring is not closed behind 
and the primary mesoblasts are widely separated from each other, or Zupoma- 
tus, in which the mesoblasts lie at one period on either side of the anus) ; and 
second, that upon the breaking up of the primary mesoblasts, which takes place 
in Lumbricus just before the invagination of the proctodeum, the mesoblastic 
bands grow around the proctodzum and join in the middle line above it. 
Hence it cannot be denied that the present position of the anus outside the 
primary mesoblastic ring may be due to secondary changes having occurred 
in the position, either of the anus, of the ring of mesoblast, or of both. 
XIII. THe TRocHOSPHERE AND THE TELOBLASTS. 
It is obvious that if the foregoing interpretation of the annelid 
embryo be accepted, the Trochosphere cannot be regarded as a 
primary larval form (representing an ancestral “ 7rochozoén”’), 
but as one which has undergone very great secondary modifi- 
cation through retardation of the trunk-region accompanied by 
early and special differentiation of the head. That it is sucha 
secondary form, appears to me to be practically demonstrated by 
the teloblasts which are so marked a feature of the larva. Proba- 
bly no one will maintain that these remarkable structures are 
ancestral in the sense of having ever existed as functional adult 
organs. Physiologically they are specially differentiated growth- 
