REPORT ON THE NEMERTEA. 95 



of cases, the ciliated canal is adapted to give the oxygenated sea-water access to the 

 hsemoglobiniferous nerve-tissue. At the same time (loc. cit., p. 35) I did not deny all sensory 

 significance to the organ, but repeated that we had not found any specialised sensory 

 epithelium in it, and could not judge of what kind the sensory impressions might be that 

 were carried by the apparatus to the animal's sensorium. Since then I have been able 

 to fix the exact mode of origin of the apparatus in at least one species of Nemertea 

 (XIV, XV), and may here recall to mind that the central lining of the canal most 

 decidedly takes its origin as an invagination of the epiblast. This invagination second- 

 arily coalesces with the brain. In these two ontogenetic data we have only very vague 

 indications. They allow of a comparison both with olfactory and with auditory pits. 

 Strange as it may seem, I do not see that the first comparison has many more a priori 

 arguments for it than the second. Otoliths, it is true, have not been found in this lobe, 

 but who can tell what purpose the minute concretions, formed by the ensheathing 

 gland-cells, and sometimes accumulated inside the lumen of the canal, may serve ? 



We have, however, to suspend our judgment. Graeffe's, Keferstein's, and Claparede's 

 observations on the existence of special otolith capsules in the Nemertea require further 

 confirmation. They may, perhaps, have mistaken the highly refractive globules in the 

 gland-cells of the posterior brain-lobe for otoliths. 



One more point may be mentioned, viz., that the comparison of the cavity of these 

 lobes with a branchial slit of Balanoglossus, &c. (cf. Bateson, loc. cit.), which I tenta- 

 tively attempted in a former paper (IX, p. 33) has to be definitely abandoned, now that 

 the epiblastic origin of the cavity has been indubitably shown by myself and afterwards 

 by Salensky (XIV, XXX), and since on this point the statements of earlier authors as to 

 the hypoblastic origin of this cavity (Barrois, &c), which led to my former suggestion, 

 have to be definitely abandoned. 



If the comparison with a gill-slit is no longer tenable on morphological grounds, this 

 in no way changes my views as to the physiological importance that must be attached to 

 the direct respiratory function of the nerve-tissue, which can nowhere be so perfectly 

 accomplished as in the posterior canalised lobe. I have no doubt, however, that in some 

 species — -more especially of Hoplonemertea — its significance as an organ of sense may 

 supersede its importance as a respiratory chamber, the haemoglobin, though present in 

 these species, being there much more diluted, at any rate colouring the brain less intensely 

 red, and the connection between posterior and anterior lobes being at the same time less 

 intimate. 



Having now discussed all those parts of the organism which we have any — though 

 sometimes even questionable — right to consider as sense-organs, I must pass on to those 

 which are of a still more dubitable nature, and which fall under the head for which the 

 further part of the superscription of this section was intended. 



Since in some cases I find them in the head and directly innervated by the brain, since 



