MORPHOLOGY OF BATHYNELLA AND ALLIED CRUSTACEA. 509 
reduction of size.” Thus, we may suppose that the absence 
of diverticula of the alimentary canal and the reduction of 
the epipodial vesicles in Bathynella are due to the fact 
that the necessary proportion of secretory, absorptive, and 
respiratory surfaces can be attained without the need for out- 
growths that are indispensable for more bulky organisms. 
Apart from questions of adaptation, however, there are other 
ways in which size greatly influences structure. As D’Arcy 
Thompson (1917, p. 33) has recently reminded us, the physical 
and mechanical conditions of growth may be profoundly 
different in a small animal from what they are in a large one. 
For example, we find in Bathyn ella and, I believe, in all 
minute Crustacea, a certain clumsiness of modelling and a 
tendency to rounded outlines in the smaller appendages such 
as the mouth-parts which may be the result of the greatly 
increased pressure due to surface tension on strongly curved 
surfaces. With regard to some other characters we can only 
dimly guess at the mechanical principles that may be involved. 
It seems to be a general rule, to which Bathynella con- 
forms, that in small Crustacea the setas on the limbs are 
fewer in number and larger in relative size than in larger 
species. It is rare, in very small Crustacea, to find any of 
the appendages produced into long multiarticulate flagella. 
Where such flagella are present, as in the antennas of the 
males of some Cumacea hardly larger than Bathynella, the 
segments of which they are composed are always much longer 
than wide. So, in the antennules and antennae and in the 
thoracic exopodites of Bathynella, the small number of 
segments and their elongate form are very striking when 
compared with the same appendages of Anaspides. 
VIII. The Classification or the Syncarida. 
If we compare the characters of the living genera of 
Syncarida it is at once apparent that Anaspides and 
Paranaspides are closely related, while Koonunga and 
Bathynella differ widely from them and from one another. 
