EMBRYOLOGY OF BDELLODRILUS 187 
now take the position within the egg membrane of the least 
resistance to their further growth. Figure 74 shows the over- 
lapping of the ends. The stomodaeum is completely formed 
and the annuli of the pharynx are visible. Figure 73 shows an 
unusual condition in the position of the ends. At this stage of 
development the animal often turns on its longitudinal axis, 
largely on account of the action of the muscles, and, instead of 
the convex side being ventral, it now becomes dorsal. This 
rotation on its longitudinal axis has no significance, as has been 
thought by previous investigators, in the later stages of develop- 
ment. The animal is extremely plastic and may assume any 
position or shape, as shown in figures 74 and 76. Figures 77 
shows the completely developed animal at the time of emergence 
from the cocoon. The number of the segments is distinct. 
This peculiar form of growth within the cocoon is merely adaptive. 
Occasionally, when the cocoon is of an unusual size, the develop- 
ing worm is less bent on itself. 
A COMPARATIVE STUDY OF DIFFERENT FORMS 
In following the cleavage cells of annelids, molluscs and poly- 
clades, one is impressed with the striking resemblances in their 
different stages of development. If this marked similarity 
alone were a sufficient criterion for a basis of classification, some 
of the most widely separated forms, when considered from the 
standpoint of their early development, would be grouped as 
closely related species. How can such resemblances in develop- 
ment be explained in animals which are so unlike in their late 
stages of growth? Are they merely the result of such mechanical 
principles as surface tension, alternation of cleavage, and pres- 
sure, or is the nature and structure of the protoplasm the com- 
mon cause? According to Driesch, ‘the striking similarity’ 
between the types of cleavage in annelids, molluscs and poly- 
clades does not appear startling and is easy to understand, 
since cleavage is of no systematic worth. However, the more 
recent investigators on cell lineage, according to Heath, look 
upon the early cleavage stages as something more than a mere 
