NOTKS AND MEMORANDA. 341 



nervous system is stated by these investigators to be formed by a 

 nerve ring, situated along the line of insertion of the velum, and 

 composed of two bands separated by the structureless lamina 

 interposed between the two epithelial layers of the velum. i 



The existence of a central nervous system of this type is described 

 in a large variety of forms from all the main groups of the Craspe- 

 dota, including examples from the Trachymedusse and the true 

 Gonophores, both Oellata and Vesiculata. The nervous rmg 

 reaches its highest development in the Geryonidse. It is, however, 

 even in its most differentiated ivtxm,not separated from the ectoderm. 

 The upper band is formed of immeasurably fine fibres with deli- 

 cate swellings at intervals. Among the fibres a fair number of, 

 for the most part, bipolar nerve cells are scattered. In the lower 

 band the nerve fibres are larger and the ganglion cells more 

 numerous than in the upper. The two bands appear to be 

 connected by delicate fibres passing through the lamina which 

 is interposed between them. 



The ectoderm adjoining both the bands is formed of but a single 

 layer of cells which may, however, be divided into two categories. 

 A. Interstitial cells. B. Sense cells. The sense cells are 

 formed of an elongated body with a delicate cilium projecting 

 from the free surface, and prolonged below into fibres which j)ass 

 into the nervous bands. There is a much larger number of sense- 

 cells connected with the upper than with the lower band. 



The peripheral nervous system is formed of multipolar gang- 

 lion cells interposed between the superficial epithelium and the 

 layer of circular muscles on the under surface of the disc. 



Multipolar garglion cells are also present in the tentacles, 

 but, as it would seem, not in the velum. The ganglion cells of 

 the disc form a complete network connected with the lower band 

 of the nerve ring, and probably also with the muscles and sense 

 cells of the tentacles. 



The nervous system of the Acraspeda has been investigated by 

 Glaus, Schiifer, and the Brothers Hertwig. The fullest descrip- 

 tion is that by the brothers Hertwig, which on the whole fairly 

 agrees with tliat by Schafer. 



The central part presents a marked contrast to that of the 

 Craspedota in that it does not consist of a nerve ring round the 

 edge of the disc, but of a series of isolated ganglia usually 

 eight in number, though sometimes more numerous. These 

 ganglia are thickenings of the ectoderm which generally take the 

 form of a ring surrounding the base of a sense organ, and are 

 formed of sense cells continued below into nerve fibres. Nerve 

 cells, similar to those in the central nervous system of the Crus- 

 ' Eiiier gives a similar accuuiit of the double uerve-ring. 



