LIGIA. 95 
that the yolk becomes surrounded by mid-gut epithelium. 
Two flask-shaped vesicles are constricted off on each side 
of the anterior end of the mid-gut. These are the 
rudiments of the hepatic tubules, which are formed by 
their backward growth, and a longitudinal constriction 
and division of each rudiment into three parts. 
The rudiments of the thoracic limbs are biramous 
(Pl. IV., fig. 8), a fact which is used in support of the 
theory that the Crustacea have descended from a schizo- 
podous ancestor. In Légia the inner limb (endopodite) 
alone develops, the exopodite being suppressed. 
The nervous system arises as a continuous whole from 
the ventral thickening of the ectoderm between the limb 
rudiments. The thoracic ganglionic rudiments are paired, 
but those of the abdominal segments are unpaired. Three 
pairs of ganglia form the supra-oesophageal ganglion, 
namely the optic, first and second antennal. ‘The sub- 
oesophageal ganglion is formed by the fusion of four pairs 

of gangha—the mandibular, first and second maxillar and 
the maxillipedal. There are rudiments of seven ganglia 
in the abdomen; rudiments also of seven pairs of 
abdominal appendages are originally formed. 
The heart is formed by the fusion of two dorso-lateral 
layers of cells, crescentic in section, and lying dorsal to the 
gut. The limbs develop successively from before back- 
wards. In the earlier stages of development the embryo 
has a dorsal curvature, but later it becomes ventral. On 
hatching, the young isopod possesses six pairs only of 
thoracic appendages, which are imperfectly segmented, 
and not setose; the cephalic region of the young animal is 
large in proportion to the rest of the animal. It leaves 
the brood pouch of the female, and after several moults 
attains the adult form. 
