THE BODY-CAVITY, ETC. 73 



The leaflets by lengthening and narrowing gave rise to the filaments. The 

 gill of Nucnla is further primitive in its free pointed posterior termination, and 

 may without further question be directly homologised with the bipectinate 

 gill of the lowest Gastropods. This last view of the Lamellibranch gill, which 

 was advanced years ago by LEUCKHABT (No. 30), has recently, owing to the 

 researches of PELSENEER (Nos. 40 and 41), MENEGAUX (No. 35), and others, 

 received great support and has become almost universally adopted. The 

 ontogenetical fact that one of the rows (the inner row) appears first and the 

 other (outer) row only much later does not, indeed, appear to be in harmony 

 with it. In tracing the gill back to that primitive form, we should expect 

 that the two rows of papillae would arise almost simultaneously. 



The rise of the gills in the form of leaves, as in Teredo and Cyclas, may, 

 according to the present state of our knowledge, best be compared to the pro- 

 duction of the branchial filaments or papillae from the ridge. We should, 

 indeed, require to understand more exactly the way in which the second 

 branchial leaf found in these animals arises. We must be careful not to 

 ascribe too great significance to the method of formation of the gills in Teredo 

 and Cyclas, because these are, as has already been shown, highly specialised 

 Lamellibranchs, and because, in the nearly related Pisidium, the leaf-like 

 rudiment of the gills is far less distinct (according, at least, to RAY LAN- 

 KESTER). These varied conditions are somewhat difficult to reconcile, and 

 their explanation is very desirable. So far, there are many indications that, 

 in the development of the Lamellibranch gills, great modifications have been 

 introduced which render it very difficult to form conclusions as to their 

 original constitution. 



F. The Body-cavity, the Blood- vascular System and the Kidney. 



The development of the closely related structures, the body-cavity, 

 the blood-vascular system and the kidney, have been investigated in 

 the Unionidaf and in Cydas, but are best known in the latter. Our 

 information on these points is due to the investigations of LEYDIG, 

 STEPANOFF, GANIN and v. JHERING, which have recently been ex- 

 tended and supplemented by ZIEGLEB. The history of the meso- 

 dennal structures, in Cycla* and the remaining Lamellibranchs has, 

 indeed, not yet been exhausted, as will be evident from the folio wing- 

 account . 



The first rudiment of these mesodermal structures appears at a 

 time when the embryo, through the development of the foot and the 

 formation of the mantle-folds passes out of the Trochophore stage, i.e., 

 at ;i stage occurring between the two depicted in Figs. 19, p. 40, and 

 .21 A, p. 44. 



In the Trochophure there is on each side of the intestine a compact 

 mass of mesoderm-cells (Fig. 19, mes) which ZIEGLER claims as the 

 inesoderm-bands. In the anterior end of each of these masses, a 



