NO. 1350. NATURAL HISTORY OF THE ISOPODA— RICHARDSON. 19 



From the anterior portion of the supra-cesophageal ganglionic mass 

 two pairs of nerves are given off to the antennas and antennules. 

 They arise independently from the brain. Some little distance back 

 of these is the place of origin of the optic nerves, which supply the 

 eyes. From the central ganglionic masses of the ventral nerve cord 

 l)ranches go to two lateral ganglionic plexuses, lying under the hypo- 

 dermis, which are the elements of the peripheral nervous system. 



The visceral nervous system consists of an anterior and a posterior 

 division. The anterior system starts from the circum-oesophageal 

 commissure and innervates the oesophagus, stomach, and liver. There 

 is a ganglion near the upper lip and one in front of the stomach. The 

 posterior system has its origin in the fused abdominal ganglia and has 

 no ganglionic differentiation. 



The reproductive organs are paired organs lying on either side of 

 the body in the region of the thorax and open by means of ducts at 

 the base of the fifth pair of legs in the female, the male duct opening 

 at the posterior margin of the seventh thoracic segment. 



Respiration is effected by means of some or all of the branches of 

 the pleopoda, these branches being thin and vascular in structure and 

 acting as gills in aerating the blood. 



4. DEVELOPMENT. 



Within the last few 3- ears, Dr. J. P. McMurrich '' has made a 

 thorough and exact investigation of the development of some of the 

 Isopods, having studied Jasra, Aselhts, Ligia, Cyinothoa^ Porcellio^ 

 and Ar)iiadillidluui. 



He proved conclusivelj'' that the segmentation of the ii^^ is centro- 

 lethical, the cells formed by the centrally situated segmentation cell 

 gradual!}' migrating to the surface and inclosing the 3'olk. The origi- 

 nal centrally situated single cell is stellate in shape, with protoplasmic 

 fibrils extending from it to the peripheral la3'er of protoplasm. The 

 ^^^^ is inclosed in two membranes, the chorion, which surrounds it 

 before the polar bodies are given off, and the vitelline membrane, 

 which is formed by the activity of the protoplasm of the ^g^^ during 

 the period occupied by the maturation of the ovum. 



The first plane of segmentation lies at right angles to the long axis 

 of the Q^y^^, the division effecting only the centrally situated cell. The 

 daughter cells thus formed remain connected by protoplasmic strands. 

 The second division results in the formation of four stellate cells, the 

 two pairs rotating through an angle of fortj^-five dcgreesuthe cleavage 

 in this stage bears most resemblance to the spiral form. The third 

 division results- in the formation of eight stellate cells. The next stage 

 is the sixteen-celled stage. In the thirty-two-celled stage the cells 



a Journal of ^Morphology, XI, 1895, i)p. G3-155 from which this account is taken. 



