MORPHOLOGY OF BATHYNELLA AND ALLIED CRUSTACEA. 505 
front by a well-marked groove from the strip of cuticle which 
is reflected to become the lining membrane of the carapace. 
On each side this strip (Text-fig. 12, I) widens somewhat and 
becomes what is clearly tlie lateral portion of the first thoracic 
tergite. For the greater part of its length it is limited 
anteriorly by the fold (Text-fig. 12, c.f .) which marks the 
beginning of the free portion of the carapace. Towards its 
lower end this fold turns forward to run horizontally where 
the lateral, or pleural, margin of the carapace overhangs the 
bases of the mouth-parts and antenna. No considerable part 
of the lateral wall of the head is exposed between the origin 
of this pleural fold and the attachment of the appendages 
except in the case of the maxilla, where a short space inter- 
venes. In front, this space is bounded by a cavity in which 
lies the palp of the maxillula (Text-fig. 12-, mx) ; behind, 
between the base of the maxilla and that of the first thoracic 
appendage, a shallow and inconspicuous groove (Text-fig. 12, 
gr.) can be traced running upwards for a little distance and 
curving forwards to join the carapace-fold. It is this short 
groove alone that can be definitely stated to mark the 
boundary between maxillary and first thoracic somites, or, in 
other words, between head and thorax. 
With exception of the short groove just mentioned, which 
I have not been able to observe in any other form, the con- 
ditions found in Grnathophausia appear to be repeated in 
all those cases where the eight thoracic tergites have been 
stated to be free from the enveloping carapace (Nebalia, 
larval Stomatopods, larval Decapods) ; that is to say, the 
cephalothoracic tergal boundary is coincident with, and is 
obscured by, the origin of the carapace-fold. Further, since 
there is reason to believe that where the first thoracic somite 
is coalesced with the head, as it is in Isopoda and Amphi- 
poda, a vestigial carapace-fold is involved in the coalescence, 
we arrive at the conclusion that Bathynella and the 
Anostracous Branchiopoda are the only living Crustacea 1 in 
1 Possibly the Copepoda should be added, but tlie case is a little 
obscure (cf . Caiman, 1909, pp. 6 and 73). 
VOL. 62, PART 4. NEW SERIES. 
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