172 BULLETIN OF THE 



pass outward through the cutis to the multicellular glands and to the 

 hypodermal cells, as already described, as well as inward {rm. mu.) 

 to the muscles. The existence of such a plexus has already been 

 shown by Andrews ('90, p. 395) for S. Gouldii. To his description, 

 which answers equally well for S. nudus, I can only add a few obser- 

 vations as to the histology of the nerve trunks. Each of these pos- 

 sesses a well defined sheath or neurogha {ngl., Fig. 5), in which discoid 

 nuclei {ngl. nl.) measuring 2 by 4.5 by 6 /;, are common. These nuclei 

 lie either inside or outside of the membrane ; they may be stained deeply, 

 and contain many nucleoli. The substance inside the neuroglia has a 

 distinct fibrillar appearance, and when these nerve stems were bent 

 upon themselves so as to be cut transversely and still extend longitudi- 

 nally within the same section, the fibrillae appear in the transverse sec- 

 tion as dots. These are also the fibres which are connected with the 

 cells of the multicellular glands {gl'" n.fbr., Plate I. Fig. 14). 



The existence of the peritoneal plexus found by Andrews ('90, p. 395) 

 in S. Gouldii could not be demonstrated in preserved specimens. No 

 doubt the examination of fresh material will show its presence in S. 

 nudus as well. 



4. Cerebral Organ. 



This interesting structure may be considered under two heads : first, 

 the canal ; and secondly, the surface next to the brain, or the cerebral 

 organ proper. 



The canal opens, as already described, on the dorsal median line, just 

 posterior to the tentacular fold {can.o.ceb., Figs. 2 and 3). From this 

 point it extends posteriad about 1.5 to 2 mm., to the anterior ventral 

 surface of the brain,' where it terminates blindly (o. ceb., Fig. 3). From 

 the marginal fold which surrounds the opening arise numerous longi- 

 tudinal ridges, which traverse the entire canal, and give it in transverse 

 section {can. o. ceb., Plate III. Fig. 26) a branched appearance. In a sur- 

 face view the walls of the canal appear thickly spotted with brown, and 

 further examination shows this to be due to the presence of large num- 

 bers of the characteristic pigment cells, which are usually seen crowded 

 in masses along the summits of the ridges {cL pig., Fig. 26). It is prob- 

 ably this canal which was found by Keferstein und Ehlers ('61, p. 47) in 

 S. tesselatus. The canal is correctly figured (Taf VII. Fig. 1, 2,u, u'), 

 but they evidently mistook its true character, since they say : " Ausser- 

 dem sieht man vom Hirn zum Tentakelkranz einen aiis zwei Ildlften 

 bestehenden, dicken Strang verlaufen, dcr dort endet, und an dem End- 



