ZOOLOGY AND BOTANY^ MICROSCOPY, ETC. 59 



Annelids, has endeavoured to detect signs of a fundamentally iden- 

 tical structure. The tube of Sahella penicillus is formed of two parts, 

 different in origin and in function ; the former, which is external and 

 accessory, is not secreted by the animal, but obtained from the sur- 

 rounding medium ; the other, which is essential and constant, is made 

 by the Annelid ; this is the tube proper. The very varying 

 characters of the former are to be explained by the great differences 

 in the nature of the surrounding medium, and this part of the tube 

 may differ considerably in its different regions. When the materials 

 for forming it are too large to be seized by the cirri the mucous 

 portion is much thicker ; at the same time it is to be observed that the 

 animal economizes as much as possible its own secretion. The outer 

 portion is very distinctly annulated transversely, apparently in corre- 

 spondence with the segmentation of the Sahella; though it cleaves 

 easily in all directions, it does not, owing to the structure of the inner 

 layer, fall to pieces. 



This inner layer is formed of a colourless hyaline substance, which 

 swells considerably when macerated in water, and becomes hard and 

 brittle by drying. During life it is perfectly flexible and very 

 resistent. Sections are most conveniently studied in salt solution, as 

 glycerine makes them too transparent. They are not modified in 

 structure by the action of alcohol. A transverse section reveals the 

 presence of concentric strata of some thickness ; these may be broken 

 up into very fine fibrils which swell, and rapidly become invisible 

 under the action of reagents. On the inner side there is a very 

 delicate granular layer, easily coloured by carmine. After macera- 

 tion in water, an external and an internal membrane can be made out ; 

 these form a kind of sheath or cuticle, which resists the reagents 

 which affect the median zone. The outer one is formed by large 

 irregular plexuses, the bands of which have a fibrillar structure ; the 

 inner layer is continuous and is made up of excessively fine fibrils, 

 crossing one another, but the majority have a longitudinal 

 direction ; among them there may be seen a large number of small 

 hyaline rod-shaped bodies, similar to the so-called tactile organs 

 which have been found in the integument of Vermes. The author 

 compares this arrangement with the nematocysts found in the tube of 

 Cerianthus by Haime. 



The greater part of the tube is formed by the median zone ; the 

 layers, of which it is composed, vary in number and size pro- 

 portionately with the thickness of the layer ; the bundles which 

 make it up give off anastomoses and prolongations, which traverse the 

 spaces in the plexus of the outer zone. Chemical structures show 

 that it is formed by an albuminoid substance, very near to, if not the 

 same as, mucin, and it appears to be secreted by special glands which 

 lie at the base of the branchial cirri. The calcareous part of the 

 tube of Serpula is regarded as the homologue of the mucous part of 

 the tube of Sahella. 



North-Sea Annelids.* — G. A. Hansen, in Norwegian and English 

 (in parallel columns), gives an account of the Annelids collected by 

 * 4to, Christiania, 1882, 53 pp. (7 pis., 1 map). 



