MARINE ISOPODA OF NEW ENGLAND, ETC. 301 



marked distinction of structure and function frequently occurs between 

 the organs homologous with the second and third pairs of legs in the 

 Isopoda. 



In the adult females of this order there is commonly formetl, on more 

 or less of the under surface of the thorax, an incubatory j)ouch for the 

 reception and development of the eggs. The outer surface of the pouch 

 is usually formed by four paii'S of lamellne attached just within the 

 origins of the second, third, and fourth, together with the first or fifth 

 jjairs of legs, and in the females of many genera, Sphccroma and Ascllus 

 for instance, these lamellae may be observed in a rudimentary condition 

 on the under surfiice of the thorax when not actually in use carrying 

 eggs or young. In Asellus, and in some other genera, they are found 

 upon the first to the fourth segments, instead of the second to the 

 fifth. In Anthura the incubatory pouch extends over only three seg- 

 ments, the third, fourth, and fifth ; and in AstaciUa it is confined to a 

 single segment, being composed of a single pair of elongated plates 

 attached to the fourth segment. In Tanais a further remarkable va- 

 riation occurs, and the eggs and young are carried in sacs attached 

 to the under surface of the fifth thoracic segment, while in the closely 

 allied genus Leptochelia the form of the incubatory pouch is normal. In 

 the Gnathiidce and Anthuridfr, according to Spence Bate and Dohrn, the 

 incubatory pouch is formed by the splitting of the integument of the 

 inferior surface of the thoracic segments in the females, and for the dis- 

 charge of the young the outer lamella thus formed further divides into 

 scales, one pair for each segment of the pouch. In Jwra, JEJpelys, and 

 probably other genera, a similar mode of development seems to occur. 



The six segments of the pleon are smaller than those of the thorax, 

 often much smaller, and frequently more or less united, sometimes 

 consolidated into a single piece with scarcely any trace of division 

 al>o\e, but the number of pairs of appendages is generally six, show- 

 ing the comijosite nature of the apparently simple organ. Of these 

 six i^airs of apijendages or i^leopods, the first five are more or less con- 

 cealed beneath the pleon, and consist on each side of a basal segment 

 bearing two lamellae (pi. lY, fig. 19 c), of which the outer is the anterior 

 when they overlap. These lamellie, at least the anterior pairs, are 

 usually ciliated along more or less of their distal margins with long 

 slender j)lumose setae. In the males of most of the genera, the inner 

 lamella of the second i^air bears, articulated near the base of its inner 

 margin, a slender stylet (pi. IV, fig. 19 b, s). This stylet seems to afford, 

 in many cases, specific and even generic characters. 



The last segment, sometimes called the telson, has its pair of append- 

 ages specially modified, and called the uropods (pi. X, fig. G3). They 

 consist in general like the pleopods of a basal segment beariug two 

 lamelUe, or rami, not being always lamelliform, and in the TanaUke they 

 are more or less segmented (pi. XIII, fig. 8G). One of these rauii may 

 disappear, as in Sphccroma and in some of the Idoteuke (pi. V, fig. '25 c), 

 where a further modification takes place, and the uropods are so articu- 



