JHECHANISM OF RESPIRATION, ETC. 789 



the branchial lamellae are strengthened by a pair of subcutaneous deeply staining 

 skeletal bars which appear to contain scattered cellular elements, but are not like true 

 cartilage, resembling rather a thickened basement-membrane. The skeletal bars may be 

 called the gill-bars, and each branchial folium 1 contains a pair of them. The proximal 

 ends of the gill-bars, where they pass into the substance of the stem, are united together 

 in couples so as to form U-shaped prongs in such a way that the two limbs of the fork 

 pass into consecutive gill-plates. 



Leaving out of consideration the gill-lamellae of the Acephala, a strictly comparable 

 skeletal apparatus has been found to exist in the prosobranchiate Gastropods and in the 

 dibranchiate Cephalopods. 



In Haliotis, Wegmann (op. cit. 1884, p. 316) says that an attentive examination of 

 a branchial folium shows that its venous border is much more resistant than its arterial 

 border. The former is rigid and sharply defined, the latter soft and undulating, the 

 difference in consistency of the two borders being due to the existence, along the venous 

 border, of an internal skeleton consisting of a hyaline stylet which attenuates towards 

 the apex of the lamella. 



In Sepia, Burne* has described a series of branchial cartilages consisting of slender 

 rods (one to each gill-plate) standing out from the branchial gland (which occupies the 

 stem or septum of the gill) and stiffening the free basal edge of each supporting mem- 

 brane. The cartilage of which the rods are composed agrees in structure with the other 

 cartilages of the body, i.e. branching cells imbedded in an abundant hyaline matrix. 



In Pleurotomaria each ctenidium 3 , according to the careful description furnished by 

 the late Martin Woodward 4 , consists of a stout axial septum bearing two sets of triangular 

 gill-plates. The gill-plates are strengthened by a pair of supporting rods along their outer 

 (i.e. venous) margins. These rods resemble those which I have found in Nautilus in being 

 flattened structures closely applied to the epidermis and enclosing between them portions 

 of the blood-space of the gill-plate. They lie along the dorso-lateral borders of the gill- 

 plates, and arch round from one plate to the next at the dorsal attachment of the 

 plates to the gill-septum. 



It will be seen that Woodward's account of the gill-bars in Pleurotomaria might 

 be applied almost word for word to the corresponding structures in Nautilus. 



The course of the blood through the gills has been described by Joubin 5 , who 

 has noted the absence of capillaries. 



1 An entire branchial folium consists of the branchial lamella proper and the basilar membrane by which 

 it is attached to the stem of the gill. 



2 Burne, R. H., " On some points in the anatomy of Sepia officinalis L. II. On the presence of a series 

 of cartilages in the branchiae." P. Malac. Soc. London, 1899, Vol. ni. pp. 53 — 56, text-figg. 



3 This is the term introduced by Professor Lankester for the true molluscan gill as distinguished from 

 other adventitious respiratory structures in certain Mollusca and from the gills of other animals. 



* Woodward, M. P., " The anatomy of Pleurotomaria beyrichii Hilg." Quart. J. Micr. Sc., Vol. 44, 1901. 



6 Joubin, L., " Recherches sur l'appareil respiratoire des Nautiles." Rev. biol. Nord France, n. pp. 409 — 428, 

 PI. vn. 1890. The branchial gland of Cephalopoda is regarded by Joubin as a blood-producing gland giving 

 origin to amoebocytes, therefore a kind of lymph-gland. He describes groups of cells (called in German 

 "Lymphheerde") in the stem and peduncle of the gill of Nautilus. I have also seen in the region of the 

 gills and osphradia and also in the siphuncle groups of large cells with reticular protoplasm which may be 

 lymph-glands. 



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