306 CEPHALOPODA. 



and thus eventually to lie within the mantle-cavity. They had 

 previously become flattened, and their surfaces folded in such a 

 way as to produce the bipectinate gills characteristic of so many 

 Molluscs (pp. 72 and 208). 



The development of the gills in Sepia was very carefully studied 

 by Joubin (No. 20). Further foldings of the leaflets take place, 

 and as these processes are repeated in the secondary leaflets, a some- 

 what complicated gill is produced consisting of three systems of folds 

 one above the other ; this is peculiar to the Cephalopoda. 



According to Joubin, the mesoderm is the chief factor in the 

 development of the gills which, by its own increase, determines the 

 growth of these organs and the modifications in their shape. The 

 gill-rudiment, when still papilla-like, was composed of the superficial 

 ectoderm-layer and a massive mesoderm (Fig. 134, k, p. 284). The 

 young gill, also, when only slightly differentiated, consists largely 

 of mesodermal tissue, in which, at a later stage, the cavities of the 

 blood-vessels form in the usual way, through the moving apart of 

 the cells. Only the larger vessels develop walls of their own. The 

 afferent (venous) and efferent (arterial) vessels, lying in the axis of 

 the gill (the middle lamella) become connected with the principal 

 blood-vessels of the body, and the afferent vessel of each gill is 

 said at its base to give rise to an auricle. In other Molluscs we saw 

 the auricles form from the coelomic sacs, like the ventricles ; we must 

 therefore accept these statements with caution. 



G. The Mesodermal Structures. 



The rise of the mesoderm, and of the organs derived from it, is 

 still very imperfectly known in the Cephalopoda, and can therefore 

 only be briefly noticed. We shall have to follow chiefly the older 

 researches of Bobretzky and Ussow, and shall have repeatedly to 

 refer to the conditions found in the adult animals. 



We have already described the first rudiment of the mesoderm 

 :as being contained in the peripheral thickening of the germ-disc 

 (Figs. 112-114, p. 247, etc., and Fig. 131, p. 280), and as lying, after 

 the differentiation of the entoderm, between it and the ectoderm. 

 Thus at first it forms a circular thickened layer, which then extends 

 both towards the centre of the germ-disc and towards its edge. In 

 this way it attains a size which, as compared with that of the 

 other layers (ectoderm and entoderm), is very considerable (Figs. 131 

 and 132). 



