XII. Enteropneusta 



For our general knowledge of the central nervous system of 

 this group we have the papers of Spengel, 1884-1894, Bateson, 1886. 



Of the development of the nervous system and the larvae the 

 work began in 1870 with the study of the so-called Tornaria larvae. 

 Bateson, 1884-5, worked out the life history of a Balanoglossus 

 form and later Spengel, '94 and Morgan, '91 and '94 gave an ex- 

 tended account of the Tornarian forms, including a good account of 

 the nervous system. Ritter, '94 and Davis, '08, gave stages in the 

 development of Tornaria and DoUchoglussus, and Herder, 1909, also 

 gave an account of development in which the nervous system was 

 included. 



In various accounts of the position and structure of the nervous 

 system especially as summarized in text books and other places, 

 there seems at times to be some difference in the descriptions but 

 I think for the most part the differences are in the way of express- 

 ing much the same idea so that no real difference is introduced. 



In all cases the nervous system is as a whole epidermal much 

 as in Phoronis and in starfish. The epithelium everywhere is more 

 or less made up of columnar cells at the surface with a deeper 

 nervous layer of fibers, in part branches from the surface cells, and 

 a few deeper cells. In places the epidermic nervous system is 

 more marked. The whole body then might be described as covered 

 ■with a plexus of nerve cells and fibers ; the thicker parts of the 

 plexus in places form the so-called nerves. The chief nerves of this 

 sort are a dorsal and ventral tract in the body region below the 

 collar with a circular band connecting these at the lower edge of 

 the collar, and a concentration of fibres about the base of the pro- 

 boscis, but the greater concentrations are in the collar itself. In 

 the dorsal and ventral surfaces of the collar just under the epi- 

 dermis is a concentration of nerve cells and fibers but the chief and 

 central concentration of nervous tissues is in the form of a thicker 

 cord running through the cavity of the collar on the dorsal side, 

 although connected with the epidermis at each end. This central 

 nervous system is continuous with the proboscis thickening in front 

 and as just described, with the dorsal and circular nerve tracts 

 behind. 



To summarize, the nervous system may be described as fol- 

 lows: 



1. General epidermal plexus continuous with other parts. 



2. Basal proboscis ring continued into the proboscis by a 

 more diffuse band. 



3. Ventral body nerve continued into ventral collar as a thin 

 layer. 



