The Ventral Structure of Tn'arthru.s. — Beecher. 93 
since from the beginning of these investigations it has not 
been possible to predict with safety the exact form and de- 
tails of any of the appendages. Even their presence has been 
more or less doubtful until revealed by a fortunate discovery. 
The paired appendages of the cfephalon will be taken up in 
their order, beginning with the most anterior, next the newl}^ 
observed characters of thoracic legs ; then the organs in the 
median line, the hypostoma, mouth, metastoma and anal 
openings. 
Close observation of the specimens thus far prepared for 
the purpose of showing the under side of the head fails to de- 
tect more than five pairs of appendages attached to the eeph- 
alon, apparently corresponding to the five typical limbs of the 
crustacean head. Considerable dithculty is experienced in de- 
termining from the ventral side of the specimens the posterior 
limit of the cephalon. The ventral membrane, which alone is 
usually visible, does not show marked evidence of segmenta- 
tion, and the observer is guided chiefly b}^ the margin of the 
cephalon, the extremities of the pleura, and obscure transverse 
lines on the axial membrane. In a few cases, the evident 
sliding or displacement of the dorsal and ventral surfaces 
further complicates the attempt to refer the appendages to 
definite divisions of the animal. 
Paired inn'rainose apjieudages. 
Anterior (Oiteinia', or anternniles. These have been described 
by Matthew, Walcottand the writer (1. c). Walcott showed 
their proximal extremities and their mode of attachment at 
the side of the hypostoma. Little more can nowV>e added ex- 
cept that they are evidently the first i)air of antennal organs, 
and correspond to the antennules of other Crustacea. The 
strong basal joint or shaft is shown in plate v, figures 9, 10, 
11, attached to the ventral side of the head at each side of 
the hypostoma, near the middle of its length. The shaft car- 
ries a single flagellum, and thus agrees witli the typical uni- 
ramose antennule of the nauplius of Crustacea. 'iMiis simple 
antennule is still present in the Isopoda, as in .Ifa/ninopsis 
tjipica. The Magella curve forward and approach, nearly 
touching as they cross the doulilure. Beyond the limits of 
the head, tliey are variously disposed, thougli usually extend- 
