THE BODY-CAVITY, ETC. 7^ 



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

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

 may without further question be directly honiologised with the bipeetinate 

 gill of the lowest Gastropods. This hist view of the Lamellihranch gill, which 

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

 researches of Pelseneeb iN'os. 40 and 41), Menegadx (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 Uyclas, ma\ . 

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

 duction of the branchial filaments or £>apillae 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 Lamellihranch 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 UniunidaH and in Cyclas, 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 Ziegler. The history of the meso- 

 dermal structures, in Cyclas and the remaining Lamellibranchs has, 

 indeed, not yet been exhausted, as will be evident from the following 

 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 Trocfwphvre stage, i.e., 

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

 21 A, p. 41. 



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

 mass of mesoderm-cells (Fig. 19, ines) which Zieglek claims as the 

 lnesoderin-liands. In the anterior end of each of these masses, a 



