348 KOREN AND DANIELSSEN ON THE 



32 q) ; these ganglia are united by means of a commissure, and 

 give rise to two other commissures (fig. 31 n, s), which connect 

 them with the pedal ganglia. They are oval, are distinguished 

 by their clear yellow colour, and send a great number of 

 branches to the foot (fig. 32 s). 



We have been unable to trace the nervous system further, all 

 parts of the body having rapidly become opake. It is also 

 about the time of the appearance of the nervous system, that 

 the first traces of the branchiae, of the siphon and of the retractor 

 muscles of the foot, are discoverable. The branchiae spring 

 from the edge of the mantle and then form a hollow cylinder, 

 which is twisted into loops ; fine cilia appear upon its inner 

 edge. Subsequently it becomes a little flattened and consider- 

 ably spread out. In its parietes, longitudinal and transverse 

 fibres are discoverable, which we regard as muscular tubules. 

 The cilia which exist in the middle of each loop have an extra- 

 ordinary length (PI. XII. fig. 8 b, c). 



After the development of the branchiae, it becomes exceed- 

 ingly difficult to make out the formation of the other organs ; 

 on the one hand, because the animal rarely protrudes from the 

 shell sufficiently to show these parts, and on the other, because 

 the mantle is greatly thickened and a large quantity of calca- 

 reous matter has been deposited in the shell. The latter has 

 taken the shape of that of a Nautilus, and when it is placed 

 under a strong magnifying power, it is observable that the cal- 

 careous matter is deposited in the form of a network with fine 

 meshes (fig. 2). The two rounded lobes diminish in volume 

 (fig. 5). The foot, lobed above, takes on more and more the 

 shape of that of the adult animal, and the operculum which 

 closes the aperture of the shell is completely developed (fig. 6). 

 The heart is, in this stage, divided into two chambers, whence 

 the great vessels arise. The lenses of the eyes are clearly di- 

 stinguishable, and we have frequently met with a single eye 

 presenting two streaks of pigment, but never with more than 

 one lens. The branchial cavity, whose internal surface is 

 covered with cilia, has become, at this period of development, 

 deep enough to enclose the heart completely. The edge of the 

 mantle, which divaricates further and further from the body of 

 the animal, is ciliated, and at the bottom of the branchial cavity 



