GILLS OF LAMELLIBRANCH MOLLUSCA. 45 



suspected, though never demonstrated (as he very justly points 

 out in citing Gegenbaur's ' Grundzuge d. vergl. Anatomie,' 

 2nd edit., pp. 540 and 552) — that the tissue -which forms the 

 substance of the Anodon's gill is similar to the form of con- 

 nective tissue which occurs abundantly in other parts of the 

 Mollusca, and in so many Invertebrates, as Avell as in the 

 Vertebrate embryo, and which the Germans know as Gallert- 

 gewebe (in England mucous tissue). It may well be termed, 

 as Prof. Ray Lankester has suggested to me, " primitive 

 mesoblastic tissue ;" or, more conveniently in this case, on 

 account of its relations to the vascular system,^ " lacunar 

 tissue." The characters of this tissue are discussed at length 

 by Posner, who very properly directs the attention of the 

 partisans of the various sides of the ''Bindegewebefrage " to 

 this elementary tissue. He might with equal propriety have 

 directed their attention to the "primitive mesoblastic tissue " 

 of a host of other Invertebrata, notably of the larger Vermes. 

 Kollman~ has also quite recently given a lengthy paper to 

 this tissue in the Mollusca from the purely histological point 

 of view — one which I shall not attempt to occupy. 



The care which Posner has given to the histology of the 

 "Najadenkieme " exclusively, and the use of osmic acid — 

 which has, according to my experience, no advantage in this 

 case over dilute chromic acid, and is not so efficient for many 

 purposes as absolute alcohol simply, have not furnished him 

 with true results as to the structure of the epithelium. On 

 the other hand, he has been thus led to adopt the view from 

 which I most emphatically dissent, that the respiratory 

 organ of the Unionidse is the prototype from which other 

 varieties of Lamellibranch gill are to be derived. He, in 

 fact, considers the Lamellibranch gills essentially as two 



' " A true blood- system or blood-lympli-system is only possible where a 

 mesoblast is developed, that is, in the Tripoblastica. In all Tripoblastica it 

 is represented by lacunse or channels, or by mere wide-setting of the cellular 

 elements of the mesoblast, between and around which the movement of a 

 fluid, so-called lymph, is possible." Primitive cell layers of the embryo, 

 ' Annals and Mag.,' May, 1873, p. 332. The name ' lacunar tissue ' is 

 suggested by M. Milne-Edwards' ' systeme de simples lacunes qui fait les 

 fonctions du reseau forme par les capillaires chez les auimaux superieurs.' 

 The nature of these lacunse and the relationship of their tissue to the walls 

 of the blood-vessels appears to have been fully divined by M. Milne- 

 Edwards in 1844. 



^ ' Archiv fiir Mikrosk. Anat.,' part 3, vol. xiii, 1876. The reader is re- 

 ferred to this paper for a full discussion of the nature and relationships of the 

 lacunar tissue. Kollman agrees with Posner in regarding it as a permanent 

 form of the embryonic connective tissue of Vertebrates. Flemming (Habili- 

 tationschrift, llostock, 1871) appears to have seen only that form of the 

 tissue in which the cell-body is swollen and the lacunse very narrow. 



