MARINE ISOPODA OF NEW ENGLAND, ETC. 417 



and a pair of simple uropods behind; eggs incubated in sacs attaclied 

 near the bases of the fifth pair of legs of the females. 



This genus is distinguished from the next by the structure of the 

 pleon and the uropods as given above, and the females are, when carry- 

 ing eggs or young, distinguished from all the other Isopoda by the wart- 

 like, or sac-like, ajipendages of the fifth thoracic segment. Usually a 

 small wart-like appendage is visible on each side of the inferior surface 

 of the thorax just within the bases of the fifth pair of legs, but the size 

 of these organs varies greatly, and in some specimens they become dis- 

 tended with eggs, extended lengthwise with the body and more or less 

 coalescent, so as to form the large, bilobed incubatory pouch, as figured 

 by Eathke. This pouch is, however, attached only to the fifth segment. 



The presence of a peculiar appendage to the fifth pair of legs in this 

 genus has been noted by various authors. Bate and Westwood figure, 

 in the second volume of the British Sessile-Eyed Crustacea, page 122, a 

 leg of the fifth pair with the attached pouch, which they " regard as a 

 branchial sac similar to those existing in the Amphipoda, and conse- 

 quently affording a proof of the nearer relationship of Tanais with that 

 order than is possessed by any other isopodous animal." They remark 

 further that "this ajipendage is wanting in some specimens, and its 

 variable existence is probably a character of specific distinction in the 

 group." Those authors have not, however, separated T. vittatus into two 

 species on this character. Stebbing * mentions a specimen with eggs 

 "as described by Rathke." Macdonaldt figures a female with an incu- 

 batory pouch, which he briefly describes as "a membranous expansion 

 or saccule under the thorax." 



Rathke's original description is as follows : "Beide Exemplare, die ich 

 untersuchen konnte, waren Weibchen und trugen Eier unter dem Thorax. 

 Diese aber, die iibrigens verhaltnissmiissig ziemlich gross waren, lagen 

 nicht, wie bei Idothea, Ligia und vielen andern Isopoden, in einer zum 

 Theil aus Schuppen bestehende Briithole eingeschlossen, sondern bilde- 

 ten zwei langlichovale, dicht neben einander liegende und an der Ober- 

 flache nur wenig unebene Massen von ziemlich betrachtlicher Grosse. 

 Jede von ihnen war zusammengesetzt aus den Eiern und einer durch- 

 sichtigen eiweissartigen Substanz, die um jene herumgegossen war, sie 

 wie ein Kitt zusammen hielt, und sie zugleich auch an die Bauchseite 

 des Leibes befestigte. Es zeigten demnach jene Massen ganz dieselbe 

 Zusammensetzung, wie die sogennanten Eiertrauben der Cyclopiden, 

 Lernaeaden und Branchiopoden." Eathke, having had only two speci- 

 mens, does not appear to have perceived the attachment of these masses 

 at the bases of the fifth pair of legs, and of course had no opportunity 

 to see them in various stages of development. A specimen belonging to 

 this genus and measuring 17 millimeters in length was obtained at Ker- 



* Ann. Mag. Nat, Hist., IV, xvii, p. 78, 1876. 

 t Trans. Linn. Soc, II, Zool., vol. i, p. 69, pi. xv, fig. 1, 1875. 

 27 F 



