118 ISABELLA M. DRUMMOND. 



and lobed, and forms almost the whole of the coil, being 

 followed only to a very slight extent by the stomach and 

 gonad. The pericardium is swollen, and the original left 

 side, which has hitherto been so narrow, widens out con- 

 siderably. In connection with this we may notice the 

 advanced condition of the primitive left kidney and the now 

 well-developed cord of cells {g.) which represents the gonad. 

 The definitive kidney {h.) is seen on the left side of the 

 section, and is easily recognisable by its slightly staining 

 and now folded walls. The kidney duct {k. d.) is cut across 

 to the right of the kidney, just where it passes below the 

 rectum (rec.) as described in the last stage. Both kidney 

 and genital ducts have now lost their primitive condition as 

 simple specialised portions of the mantle cavity, and run for- 

 ward in the roof of the latter as well-defined ducts, parallel 

 to and to the right of the rectum, opening somewhat behind 

 the anus. 



The mantle cavity now extends very Ioav on the left side, 

 especially posteriorly, so that it is just cut in the section 

 represented in fig. 17, the difference between anterior and 

 posterior regions being much more accentuated than was 

 formerly the case. The relations of the visceral connectives, 

 as noticed already, though in a less degree in the last stage, 

 are deeply affected by the asymmetrical growth of the mantle 

 cavity, and as posteriorly the mantle cavity appears to lie 

 wholly on the left side and with its floor almost vertical, so 

 in the region just anterior to the commissure the two con- 

 nectives lie almost in one vertical plane. A discussion of 

 these relations may conveniently be left till the whole 

 question of torsion is taken into consideration, but the twist 

 of the connectives now remains to be described. As is well 

 known, the two connectives are bilaterally symmetrical when 

 they leave the pleural ganglia, almost immediately the right 

 passes below, the left above the oesophagus, each to the 

 opposite side of the body, after which they again resume 

 their bilaterally symmetrical disposition, only that now on 

 the right side is the original left, and that now on the left is 



