240 Prof. M'Intosh's Notes from the 



the functions of both are equally -well performed. The 

 Nemertcan brain, as in many Polyclisets, is distinctly isolated 

 from the tissues outside it, and the same may be said of the 

 main trunks in that group. Here, in what is considered to 

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

 centre and main trunks being hypodermal, as are the cords 

 in the majority of the Polychaets. In Owenia this belt 

 agrees in minute structure with that surrounding the central 

 system, and occupies a corresponding position. 



With the disappearance of a central fillet in the dorsal 

 arch of the body-wall a slightly pale band is noticeable in 

 the hypoderm of the region, yet that layer passes to the 

 basement-tissue (which stains) uninterruptedly, 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 hypoderm becomes more pronounced, 

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

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

 dorsal arch. It shows both fine transverse and vertical fibres 

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

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

 cells and areolae 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, 

 nemoglia, and neurilemma of the ordinary Polychset ganglia 

 are not distinguishable. Prom end to end in section the 

 tissue has a uniform 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 deeply 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 



