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 Ligia 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 ganglia — 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. 



