■74 LAMELLIBRANCHIA. 



cavity arises which soon, by the regular arrangements of its cells into 

 an epithelium, assumes the form of a vesicle. This is the paired 

 rudiment of the pericardium. 



The rise of the paired pericardial vesicles out of the bilateral 

 mesoderm-rudiment so nearly resembles the formation of the primitive 

 segments in the Annelida and the Arthropoda that we must regard 

 the pericardial vesicles as coelomic sacs and their cavities as the 

 secondary body-cavity. The coelom in the Lamellibranchs, however, 

 only attains a very small size, and the definitive body-cavity which 

 contains the organs arises independently of the former as a pseudocode. 

 The view that the pericardial sacs must be regarded as the coelom 

 rests chiefly on the fact that the kidney shows the same relation 

 to this cavity (Fig. 3.2) as do the nephridia in the Annelida to the 

 cavities of the primitive segments (secondary body-cavity). This 

 relationship is very early developed in the embryo of Cyclas. 



The kidney (organ of Bojanus). Behind the pericardial vesicle, 

 the mesoderm-cells soon become grouped in the form of a tube, the 

 lumen of which communicates with the cavity of this vesicle. This 

 tube, which at first runs upwards, and then again bends downwards, 

 is the rudiment of the organ of Bojanus (Fig. 21 A, n, p. 44). ^ Its 

 upper end, which opens into the pericardial vesicle (Figs. 21, 32) is 

 lined with cilia. The resemblance thus brought about between the 

 organ of Bojanus and a nephridium is heightened later when the 

 lower end of the canal fuses with the ectoderm and communication 

 with the exterior is thus established (Fig. 31, n„). 



From Ziegeer's description, it is not clear whether the formation of the 

 efferent duct takes place through the direct fusion of the lower end of the rudi- 

 ment of the kidney with the ectoderm, or whether an invagination of the ecto- 

 derm takes part in it. Ziegler's statements on the whole support the first 

 hypothesis, which also agrees with the manner of formation of the nephridia 

 in the Annelida as described by Bergh.* But since, as we shall see, in the 

 ( Gastropoda and also in the Annelida (Vol. i., p. 297), an ectodermal invagination 

 takes part in the formation of the nephridia, this question cannot here he 



decided. 



The statements which have been made as to the rise of the kidneys as mere 

 depressions of the ectoderm (Ray Lankester, Ganin) must be considered as- 

 refuted, especially as the morphological agreement of the organs with the 

 nephridia of the Annelida points to a similar method of formation. We are 

 indeed led to look for a still closer relation of the nephridia, when forming, 

 with the coelomic sacs, and such a relation will be found in the Gastropoda. 



* R. S. Bergh. Neue Beitrage zur Embryologie der Anneliden. I. Zur 

 Kntwicklung und Differenzirung des Keimstreifens von Lumbncus. Zatscn. 

 r. wiss. Zool. Bd. 1. 1890. 



