Development of the Pectinibranchiata. 361 



been commenced. The head and back become more and move 

 visible, and covered with fine ciha; we also perceive cilia on the 

 tentacles, which have become longer. The eyes have acquired a 

 more conical form, and we sec the lens in them distinctly. The 

 proboscis and the tongue are completely developed, and on the 

 latter we see the armature as described by Lebert and Loven. 

 The salivary gland>i are now large enough to enable us perfectly 

 to trace their excretory duct, which follows the oesophagus 

 upwards. The siphon, furnished with cilia, is then clearly 

 perceptible. The foot has changed its form, and become 

 longer; from its upper part two rounded lobes arise. As 

 regards the structure of the foot, it is composed of a multitude 

 of primary, cylindrical, muscular tubes, which are also varicose, 

 and cross each other in all directions, without, however, form- 

 ing a mass. In the interior of the tubes we observed neither 

 nucleus nor cells. 



In this period of development the nervous system becomes 

 tolerably visible. We observe the two large cerebral ganglia 

 (PI. XVII. fig. 1 a, a), which are of an oval form, and between them 

 two smaller ganglia [b, b). From the lower part of each lai'ge 

 ganglion a short and thick nerve arises [h), which joins the 

 branchial ganglion {d), and from the upper part a more delicate 

 and rather longer branch, which runs to the eye (/,/). The 

 small cerebral ganglia are round, and about half the size of the 

 large ones. Each of them gives off a slender branch [k, k) to the 

 auditory organs, and another, a little stouter {g,g), to the two 

 pedal ganglia [c, c). The latter are of an oblong form, and it is 

 in their broadest part, which is turned towards the cerebral 

 ganglia, that the two nerves just mentioned lose themselves ; 

 these two nerves start from the smaller cerebral ganglia. From 

 the broadest part of each [pedal] ganglion starts a pretty strong 

 nervous branch {i, i), which meets on the left with the branch 

 which the large cerebral ganglion sends off to the branchial 

 ganglion; to the right this same branch meets with the bran- 

 chial ganglion, but without joining it. Nearly at the middle of 

 each pedal ganglion a nerve [m, m) originates, which runs to the 

 lobate foot, and at this point forms a small ganglion {/,/), from 

 which three branches (o) arise. The narrowest part [of the 

 pedal ganglia], on the other hand, gives off several branches 

 (/?, n) to the part of the foot wdiich is furnished with an oper- 

 culum. The branchial ganglion is of an oblong form ; its largest 

 part turns upwards, and it is in this that the three branches, to 

 which we have referred above, lose themselves, so that this gan- 

 glion is in communication with the two large cerebral ganglia, 

 and with the pedal ganglia. From its narrower part, which goes 

 downwards, originates a thick nervous trunk (p), which termi- 



