294 A. T. MASTBRMAN. 



detail the appearances which he considered to be connected 

 with the nervous system. He noticed, on the dorsal surface 

 of the pre-oral hood^ a " horn-shaped protuberance/' and, 

 posterior to this, a wart-like thickening. This latter is the true 

 nerve-ganglion, and the former is the apical sense-organ, but 

 they were not recognised by him as such. On the contrary, 

 he describes a swelling (Wulst) on the ventral surface of the 

 pre-oral lobe, which he conjectures to be the ganglion. Ra- 

 diating from this, he says, are found a number of lines, and 

 he indicates the course of these lines, and even figures the 

 nervous system, as it appears when isolated (fig. 6). His 

 " Wulst " is the aperture of the subneural gland, to be de- 

 scribed later, and the radiating lines and commissures are, at 

 any rate in large part, the mesenteries separating the cavities 

 of the pre-oral lobe and the collar, together with the mesentery 

 bounding the vascular space below the true nerve-ganglion. 

 A comparison of PI. 18, fig. 3^ with Wagener's figure will make 

 this clear. 



One can readily imagine how an incautious inspection of 

 the larva intact might cause this mistake, but how the observer 

 referred to managed to " isolate " such a heterogeneous and 

 unconnected collection of structures is not easy to understand. 



Caldwell (2) speaks of the ectoderm becoming thickened in 

 two regions : 



1. In the pre-oral lobe. 



3. In the form of a post-oral ring round the mouth. 



The former, he states, becomes the future ganglion, and the 

 latter the circum-oesophageal nerve-ring. Again, in the fully 

 developed larva he remarks that " the nervous elements of 

 the ectoderm of the pre-oral lobe in all species are concentrated 

 into a ganglion. In some species a large number of nerve- 

 fibres pass forwards from it to a sense-organ.^' 



The central nerve-ganglion (figs. 1 — 3, 13, 14, 15, 20, 23, 

 28, n,g.) is a lenticular mass lying on the mid-dorsal line at 

 the base of the pre-oral lobe. In fig. 1 it is seen to be at the 

 front end of the body, but when the hood is directed forwards 

 its true situation at the base of the pre-oral hood is evident 



