xxxii Journal of Comparative Neurology. 



the peripheral portion consists only of cells. On the 9th day the interra- 

 dial nerves to the tentacles are formed, and on the 17th nerve branches 

 arise from the median ventral nerve to the primary pedicels By the 8th 

 day the separation of the entire nervous system from the ectoderm is com- 

 plete. No indication of an auditory organ was discovered. 



Neuroblasts in Arthropod Embryos. 1 



In Xiphidium the nervous system arises early in the form of paired 

 thickenings of the ectoderm. The lateral cord consists, in early stages, 

 of small cells with elongate-oval neuclei, and four sets of larger cells with 

 pale spherical nuclei, the latter being the neuroblasts proper. There are 

 eight longitudinal rows of these neuroblasts. These cells proliferate and 

 each soon surrounds a pillar of smaller elements - the future ganglion 

 cells. The dotted substance makes its appearance in the bases of the 

 lateral cords, which are separated by a pyramidal mass of cells, the median 

 cord. 



The median cell neuroblasts arise intersegmentally, but sonn move 

 forward between the two connectives, and finally come to lie just back of 

 the posterior commissure of each segment. Later it is incorporated in 

 the posterior part of the ganglion. 



The Nervous System of Echinuderms.^ 



The following summary is derived from the American Naturalist : 

 "The nervous system of the star-fish is differentiated into two systems, 

 the condensed and the diffused, between which, however, one finds it 

 difficult to draw a sharp distinction. The functions are, however, now 

 specialized in this large group, for we have shown that the two nervous 

 fields have a distinct and peculiar functional aspect. Each presenting a 

 distinct activity, they divide between themselves the work which is apjiro- 

 priate to the life of relation. Owing to the close connection which e.xists 

 between them, the systems can aid each other and work together fur the 

 accomplishment of the multiple manifestations induced by the different 

 conditions external and internal, by the sensations arising from stimuli, 

 and by reaction. To the diffuse system is attributed, above all, the role of 

 perception and of sensation. It is the one which gives the organism a 

 notion of its position and which informs it of its state of equilibrum. It is 



1. W. M. Wheelkr. .Tourn. Morphology IV, 1891, p. :i:i7. 



2. DEMooRaiul Chapeaix. Tidsch. NederlancL Dierk, Vcreeii, III. 



