t?40 Prof. M'Intosh's Notes from the 



the functions of both are equally well performed. The 

 Ncmcrtean brain, as in many Polycliiets, is distinctly isolated 

 from tlic tissues outside it, and the same may be snid of the 

 main trunks in that j;ronp. llcre^ in ■what is considered to 

 be a hiiiher series, the opposite condition prevails, the nerve- 

 centre and main trunks beinji,- hypodernial, as are the cords 

 in the majority of the Polychicts. In Owenia this belt 

 a<>;rrcs in minute structure with that surrounding the central 

 system, and occupies a corresponding position. 



AVith the disappearance of a central fillet in the dorsal 

 arch of the body-wall a slightly ])ale band is noticeable in 

 the hypoderm of the I'egion, yet that layer ])asses to the 

 basenuMit-tissue (which stains) unintcrriiiitedly, a series of 

 the ends of severed fibres being grasped in spaces bounded 

 by reticulations between the basement-tissue and the ad- 

 joining circular muscular fibres. Then the pallor of the 

 inner portion of the hyj)oderm becomes more pronounced, 

 and in the next section or two (Fl. YII. fig. 1) a distinct 

 nervous layer, as in Cephaloiliscus, stretches along the mid- 

 dorsal arch. It shows both fine transverse and vertical fibres 

 or strife, and minute granules occur next the basement-tissue 

 (PI. YII. fig. 2). It fades on each side into the ordinary 

 cells and areohe; of the hypoderm, which likewise continues 

 to the surface externally without evident break. The nerve- 

 tissue, in short, is marked by no hard-and-fast line from the 

 hypoderm, but is traversed by its fibres, and the neuropile, 

 ncmoglia, and neurilemma of the ordinary Polyeheet ganglia 

 are not distinguishable. From end to end in section the 

 tissue has a uuiform structure, and where, for instance, it is 

 separated from the basement-layer only projecting vertical 

 fibres and granules appear. Certain granules occur at its 

 outer border next the deejjly stained cells and granules of 

 the hypoderm, but these could not be associated with the 

 nerve-band, the finely fibrillar edge of which coursed evenly 

 along. In succeeding sections this great nerve-band stretches 

 downward at the sides, becomes more distinctly differentiated 

 from the hypoderm externally and the basement-tissue in- 

 ternally, and then a slight narrowing of the mid-dorsal arch 

 takes place, the lateral extensions being thicker. The mouth 

 is still divided ventrally in these sections, and the nervous 

 expansion extends over the entire arch of the body-wall with 

 the exception of a comparatively short region of the ventral 

 edge of the lateral lip, the thickest layer being lateral, for 

 the dorsal is now diminishing. A narrow layer, apparently 

 of basement-tissue, occurs, as indicated, simultaneously in 

 the sections external to the hypoderm lining the mouth, and 



