126 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 
telson belongs, are the sixteen free segments of the body, which are homonomous or of uniform 
size, the first and second, however, being soldered together; the absence of a true carapace; the 
seven pairs of schizopod-like legs, the first pair spined and raptorial, slightly reminding one of 
those of Squilla; the second pair also spined; the antenn of both pairs are long and slender, the 
two flagella of the first pair being very unlike any sessile-eyed or edriophthalmatous Crustacean ; 
the six pairs of abdominal feet, which are long, slender, and with a general resemblance to those 
of the Schizopoda; the broader, oar-like swimming ramus, ending in long sete. Any doubts as to 
the macrouran affinities of the Synearida are removed by an examination of the long, acute telson 
and last pair of abdominal appendages; the Sarin are biramous, the divisions flattened from 
above downwards, so that they with the telson serve, as in schizopods and shrimps, for propelling 
the body backwards when the animal is disturbed. 
We should regard the Synearida as the lowest group or suborder of Thoracostraca, but much 
nearer the Schizopoda than the Cumacea; they form a connecting link between the Amphipoda and 
Thoracostraea, but at the same time in Gietr most essential characters stand much nearer to the Schi- 
zopoda than the Amphipoda; the lack of a carapace, even a rudimentary one, and the homonomous 
segmentation, causing them to bear a resemblance to the Edriophthalma, which they would not 
otherwise present. The Syncarida may be regarded as the homotoxial equivalents of the Decapoda, 
Schizopoda, or Stomapoda. To the Isopoda, Acanthotelson presents superficial resemblance, due 
to the slightly vertically compressed body and the homonomous segmentation. The Edriophthalma 
(Arthrostraca of some late authors) are defined by Claus as “ Malacostraca with lateral sessile 
eyes, usually with seven, more rarely with six or fewer separate thoracic segments, and the same 
number of pairs of legs, without a carapace,” but this definition does not express those differences 
in the form of the antenne, the thoracic legs, and abdominal appendages, especially those of the 
end of the urosome or abdomen, which are characteristic of the sessile-eyed Crustacea as distin- 
guished from the Thoracostraca. 
From the Isopoda, in which the body is usually broad and vertically flattened, with seven free 
thoracic segments, while the abdominal legs are lamellar and closely appressed to the short abdo- 
men, our Acanthoteson plainly differs in the long bi-flagellate Decapod-like first antenn, in the 
long homonomous segments of the abdomen, and the schizopodal abdominal feet, and especially the 
Schizopod-like telson and last pair of feet, adapted, as in the shrimps, for striking the water from 
above downwards. 
The Amphipoda are, in general, characterized by their laterally compressed body, with lamel- 
late gills on the thoracic feet, and an elongated abdomen, of which the three anterior segments 
bear the swimming feet, while the three posterior bear posteriorly-directed feet, adapted for 
springing (Claus). Now, if Acanthotelson is not an Lsopod, still less should it be regarded as 
related to the Amphipoda. The first antenne are entirely unlike those of any known Amphipods, 
the latter having a very short accessory flagellum; the second antennwe of Acanthotelson are 
strictly decapodous in appearance and very different from those of the Amphipoda, whereas in 
Gammarus the scape is as long as the flabellum. Although there are seven free thoracic segments 
in Acanthotelson as well as in Gammarus and other Aimphipoda, those of Acanthotelson are not 
compressed any more than in the Schizopoda, and there are no traces of epimera; on the contrary, 
the free edges of the thoracic and abdominal segments are much as in the Schizopoda and Caridea. 
The thoracic appendages of Acanthotelson are, on the whole, like those of the Stomapoda and 
Schizopoda. We cannot detect any traces of mouth-parts, mandibles with their palpus, or maxilla; 
but the thoracic legs do not present any close resemblance to those of the Amphipoda, the first 
pair being as much, if not more, like those of Squilla than any Amphipod with which we are 
acquainted, while the three posterior pairs, which are in form and size like those in front, entirely 
differ from those of Gammarus and most other normal Amphipods, in which the basal joint is very 
large and triangular. Turning to the abdomen, the difference in that of Acanthotelson from that 
of the Amphipods is still more marked. The first five pairs of uropoda, or abdominal appendages, 
are, in Acanthotelson, all formed apparently on the same plan, not essentially different from those 
of Schizopods, while the last pair are flat and on the same plone as the telson and intimately asso- 
ciated with the latter; in short, these parts are formed on a truly macrurous plan and most approach 
those of the Schizopods, in which the telson and rami of the last pair of feet are narrow and more 
