BURNE : ANATOMY OF HANLEYA ABYSSOKUM. 5 



the importance of this organ, especially for the determination of the 

 true nature of the gills, he apparently did not at the time enter upon 

 an examination of its detailed structure. Two years later the matter 

 was investigated by ISela Haller ^ in the course of an exhaustive 

 inquiry into the anatomy of two species of Chiton (C. siculus, Gray, 

 and C. fascicular is, Poli). He was unable to confirm Spengel's 

 suggestion. In transverse sections of the gills he observed that, 

 although the epithelial cells covering the outer walls of both branchial 

 vessels were sligbtly larger than those upon other portions of the gill, 

 and were provided with enormous cilia, yet there was no marked local 

 epithelial thickening characteristic of an osphradium, and no pigment 

 cells. The absence of the latter, and the strongly pigmented nature 

 of the blood, led him to suppose that probably Spengol had been 

 deceived by the pigmented appearance given to the living tissues by 

 the blood. This is practically the last we hear of Spengel's osphradium 

 on the outer surface of the gill. The subject was re\'ived again in 

 1891 by Blumrich,'- who devotes some pages to the description of an 

 organ, which he regards as the osphradium, situated this time not 

 on the outer but on the inner side of the gill. The organ consists 

 essentially of two ridges of lofty epithelium extending from the first 

 to behind the last gill : one ridge (parietal) is situated on the body- 

 wall ; the other (paraneural) beneath the lateral nerve-cord, extending 

 somewhat on to the surface of each gill. The epithelium consists of 

 large glandular cells and hair cells ; the latter are specially concen- 

 trated in certain positions, forming sensory knobs ; in some species the 

 ridges may vanish, leaving only the knobs ; the hair cells are in 

 communication with the lateral uerve-cords. The lining of the genital 

 duct is continuous with the paraneural ridge. Tbese tracts of 

 modified epithelium were originally described by Haller,^ but were 

 considered by him to be glandular in function and not sensory. The 

 true meaning of this epithelium is doubtful, for Haller ^ still persists 

 in his original view, laying stress on the close relationship between 

 the ridges and the genital ducts, and pointing out that most probably 

 it is homologous to the hypobranchial gland of Prosobranchs. Thiele ^ 

 also is unconvinced by lilumrich's observations. On the other hand 

 Simroth,® taking an impartial survey of the question, considers that 

 Elumrich's interpretation is probably correct ; and this view has also 

 been adopted by Lang,' although he does not discuss the matter. 



During the spring of the present year, three specimens of Ranleya 



' B. Haller, " Die Organisaliou der Chitoueu dor Adria." Ft. 2 : Arb. lust. 

 Wien, V, 1883, p. '1%. 



^ J. Bluinrich, " Das Iiiteguineut der Chitoueu": Zeitschr. Wis*. Zool., Hi, 1801, 

 p. 460. 



^ Loc. cif. p. 21. 



* Haller, " Beitrage zur Keuutuis der Placophoren " : Morpli. Jahrb., xxi, 1894, 

 p. 34. 



^ Thiele, " Beitriige zur Keuutuis der Molluskeu": Zeitschr. Wiss. Zool. liii, 

 1892, p 586. 



^ Bronu's Klassen und Orduuugeu des Thier-reichs, Bd. iii, Mollusca, 1894, p. 262. 



' A. Laug, Lehrbueh der vergleieheudeu Anatomie, 1888-94, p. 744. 



