126 MEMOIRS OF THE NATIONAL ACADEMY OP SCIENCES. 



telson belongs, are the sixteen free segments of the body, which are homouomoiis 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 antennae 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 setae. Any doubts as to 

 the macrouran afflnities of the Syncarida are removed by an examination of the long, acute telson 

 and last pair of abdominal appendages; the appendages are biramous, the divisions flattened from 

 above downwards, so that they with the telson serve, as in schizoi^ods and shrimps, for propelling 

 the body backwards when the animal is disturbed. '" 



We should regard the Syncarida 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 

 Thoracostraca, but at the same time in their most essential characters stand much nearer to tbe 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 Edriophthahna, which they would not 

 otherwise present. The Syncarida may be regarded as the homotoxial equivalents of the Decapoda, 

 Schizopoda, or Stomapoda. To the Isopoda, Acanthotelsou presents a superficial resemblance, due 

 to the slightly vertically compressed body and the homonomous segmentation. Tbe Edriophthalma 

 (Arthrostraca of some late authors) are defined by Clans 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 antennae, 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 Acanthotesou plainly difters in the long bi-flagellate Decapod-like first antennae, in the 

 long homonomous segments of the abdomen, and the schizopodal abdominal feet, and esi)ecially 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 postei'iorly-directed feet, adapted for 

 springing (Clans). Now, if Acanthotelsou is not an Isopod, still less should it be regarded as 

 related to the Amphipoda. The first antennae are entirely unlike those of any known Amphipods, 

 the latter having a very short accessory flagellum; the second antennae of Acanthotelsou are 

 strictly decapodous in appearance and very different from those of the Amphipoda, whereas in 

 Gammarus the scape is as long as the tiabellum. Although thei-e are seven free thoracic segments 

 in Acanthotelsou as well as in Grauimarus and other Amphipoda, those of Acauthotelson 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 Schizoi)0(la and Caridea. 

 The thoracic appendages of Acanthotelsou are, on the whole, like those of the Stomapoda and 

 Schizopoda. We cannot detect any traces of mouth-parts, mandibles with their pali)us, or maxillae; 

 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 Ami)liipods, in wliich the basal joint is very 

 large and triangular. Turning to the abdomen, the dift'erence in that of Acanthotelsou from that 

 of the Amphipods is still more marked. The first five pairs of uropoda, or abdominal appendafjes, 

 are, in Acanthotelsou, all formed apparently on the same plan, not essentially different from those 

 of Schizopods, while the last pair are flat and on the same pLine 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 



