ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 205 



an ectodermic thickening. We may therefore conclude that the organ 

 of Bojanus, like the nephridia of Annelids, is formed partly by ecto- 

 dermic and partly by mesodermic cells. The resemblance is the more 

 complete, if, with Grobben, we regard the pericardiac cavity of molluscs 

 as the homologue of the coelom of Annelids. The disappearance of one 

 pericardiac vesicle and one organ of Bojanus is evidently correlated 

 with the general asymmetry of the body in these molluscs. If the 

 pericardiac cavities are coelomic in nature, then the inner walls of the 

 pericardiac vessels correspond to the dorsal mesentery of Annelids. 



Sucker on Fin of Pterotrachea.* — Mr. J. W. Fewkes points out, 

 not with a view of claiming priority, but to corroborate succeeding 

 writers, that in 1883 he observed — as Paneth and Grobben have since — 

 that the sucker on Pterotrachea is not confined to the male of this 

 Heteropod. 



5. Lamellibranchiata. 



Histology of Najadae.f — Herri. Apathy publishes a summary of his 

 Hungarian monograph on the histology of Najadse. 



I. Blood. The corpuscles (e. g. of Unio) have manifold forms, and 

 are not characterized by few and short processes as Flemming described. 

 A special form is distinguished by large nucleus, almost absent pro- 

 cesses, and absence of tendency to unite with others. The nuclei of 

 the corpuscles were observed in indirect division. The pericardial fluid 

 is not blood, though corpuscles may wander into it. 



II. Connective tissue. The hyaline intercellular substance with its 

 clefts is characteristic. Physiologically, the elements may be distin- 

 guished as (a) proper connective tissue cells, producing the intercellular 

 substance ; (b) mucous cells without share in the latter. A nucleus is 

 present in all these. A portion of the fine fibres on the walls of the 

 blood-vessels belongs to the connective tissue system. Fine fibrils are 

 also demonstrable in the hyaline matrix, especially if celloidin be used 

 for imbedding. The cells without processes, which Kollmann describes 

 as " Hautchenzellen," are more or less altered and shrivelled cells, which 

 no longer fill their original space in the matrix. The mucous cells 

 may retain their mucus within their membrane (Langer's Bliischen), or 

 empty it by a distinct canal (mucus-cells proper). There could be no 

 doubt as to the intactness of the vesicular cells, which are certainly not 

 " lacunas." There is a continuous transition between the latter and the 

 glandular mucous cells with distinct openings. The rmicus secretion, 

 and in part the shell secretion, pertain to the connective tissue system. 



III. Epithelium. This is always in one layer. There is no proper 

 endothelium. Every epithelial layer has a cuticle, even that which 

 bears cilia. The cuticle is a cementing substance. The superficial 

 pigment differs from that of connective tissue, glands, or nerves, in being 

 more finely granular and much less soluble in alcohol or ether. The 

 basal portions of the cilia are connected with the cellular protoplasm by 

 narrow processes penetrating the cuticle. Engelmann's conclusions as 

 to the histology of the cilia are considerably modified. Flemming's 

 tactile brush cells occur over the whole surface of the body. They are 

 really double ; the spindle-shaped, darkly pigmented, superficial portion 



* Zool. Anzeig., xi. (1S8S) pp. G4-5. 



t Naturh. Abh. Ung. Akad., xiv. Cf. Biol. Centralbl., vii. (1887) pp. 621-30. 

 1888. Q 



