1908.] ASPIDOBRANCH GASTROPOD MOLLUSC'S. 829 



In order to avoid repetition of details I will joass briefly over 

 the rest of the nervous system of SejJtaria. In all essential 

 features it resembles the nervous systems of Nerita and the 

 tropical Neritinae, which I shall describe in greater detail in the 

 subsequent part of this paper. I need only say here, because 

 Bela Haller has thrown doubt upon these points in his description 

 of the nervous system of Nerita ornata, that there is a well- 

 defined labial commissure in Septaria, and that I can find no 

 trace of transverse commissures, posterior to the main anterior 

 commissure, between the pedal cords in this genus. 



The position and structure of the subpallial sense-organ or 

 osphradium in the Neritidpe has been correctly described by 

 Bernard (3) and Thiele. but the latter author throws doubt on 

 its homology with the true osphradium of other Mollusca, and 

 other authors give doubtful or incorrect descriptions of it. In 

 Septaria this sense-organ is easily distinguished in transverse 

 sections as a prominent ridge of epithelium running forward from 

 the anterior end of the left suspensory fold of the ctenidium 

 along the roof of the mantle-cavity and ending only a short 

 distance behind the thickened anterior edge of the mantle. It 

 lies almost in the angle formed by the union of the mantle with 

 the left columellar muscle, and its position at the inhalant side of 

 the mantle-opening is consistent with the function usually 

 attributed to an osjDhradium, that of a sense-organ for testing the 

 quality of the water before it passes over the ctenidium. The 

 cells covering this ridge are higher, their nuclei are more closely 

 crowded together and stain more deeply than those of the 

 adjacent mantle epithelium. Under a high power of the micro- 

 scope the epithelial ridge can be resolved into three parallel 

 strips. The two outer strips {a-h and h-c in fig. 17) consist 

 solely of columnar epithelial cells Avith granular cytoplasm and 

 rather large nuclei. The free ends of these cells bear cilia which 

 in the groove shown on the lower side of fig. 17 are longer than 

 elsewhere. The central strip {b-h in fig. 17) is largely composed 

 of the same elements, but its character of a sensory epithelium is- 

 well shown by the presence of a number of attenuated sense-cells, 

 interspersed between the larger columnar cells. The nuclei of 

 the sense-cells are smaller and more elongated than those of the 

 columnar cells ; their cell-bodies stain deeply in carmine, and in 

 many cases it can be seen that their inner ends are prolonged 

 into fine fibrils which ti-avei-se the thin layer of muscle-fibres and 

 connective tissue underlying the osphradiian and pass into the 

 osphradial ganglion. The size and position of this ganglion at 

 the place where it is joined by the large branchio-pallial nerve 

 are shown in fig. 17. 



The respiratory and circulatory systems have been worked out 

 in some detail by Lenssen in Neritina fluviatilis, and their 

 arrangement is similar in Septaria, but allusion must be made 

 to one or two points in which Lenssen's account is defective. 



The ctenidium in Heptaria is an elongated triangular organ, its 

 free pointed extremity directed forward and to the right; its 



