5-08 
W. T. CALM AN. 
of Anaspides; while, as in Palaeocaris, it co-exists with 
a clear demarcation between the head and the first thoracic 
somite. As was previously pointed out this mandibular 
groove is, in all probability, to be identified with that named 
by Sars “ cervical sulcus 33 in the Mysidacea, with the trans- 
verse cephalic groove of the Anostraca and Conchostraca,. 
and with the anterior transverse groove of the carapace in 
Apus and other Notostraca. It is possible, but much less 
certain, that it corresponds to a part at least of the 
“ anterior cervical groove” of Decapoda. As regards its- 
morphological significance, its position at the limit between 
the naupliar and post-naupliar regions of the body suggests 
that it may be of great phylogenetic antiquity and importance. 
It is quite possible, however, that it may have rather a 
mechanical and function^ meaning. In the Anostraca the 
bottom of the groove is thickened to form a more or less 
continuous chitinous bar connecting across the dorsal surface 
of the head the points of articulation of the two mandibles. 
The thickening is most marked, as it is in Bath yne 11a, at 
the ends of the groove ; and there can be little doubt that, 
whatever its origin, this groove hns, at least in these two- 
cases, the function of giving the necessary support for the 
articulation of the proximal condyle of the mandible. 
VII. The Degeneracy of Bathynella. 
The structure of Bathynella, as compared with that of 
its immediate allies, is obviously, in many respects, simplified 
or degenerate. Some of the evidences of degeneration are no- 
doubt correlated with the habitat of the animal, particularly 
the absence of eyes, which is almost universal in animals that 
inhabit subterranean waters. It is quite likely, however, 
that the simplification of structure is in great part a direct 
consequence of unusually small size. Lankester (1880, p. 51) 
long ago pointed out that “ the needs of a minute animal are 
limited as compared with those of a large one,” and ho 
enumerated as one of the causes of degeneration <c excessive 
