108 HYATT, 



The form of the nerve-mass of any of the remaining 

 genera, in a direct view, differs greatly from that of Fred- 

 ericella (Figs. 9, 12), instead of being elliptical it is elon- 

 gated, and has a depression on the posterior side dividing 

 the lateral or ganglionic centres of distribution from each 

 other. This change is in great measure due to the differ- 

 ent position of the mass. For instead of being upright 

 and parallel to the oesophagus as in Fredericella the pos- 

 terior end is rotated outwards until the whole becomes hor- 

 izontal as in Pectinatella (PI. 11, fig. 1, 8'). During this 

 rotation the epistomic nerve-trunk passes from the anterior 

 to the posterior end along the side corresponding to the 

 ventral side of the nerve-mass in Fredericella (PL 11, 

 fig. 1, and figs. 10, 11, T'). The lophoric and the polypi- 

 dal nerve-branches separate from each other forming four 

 distinct, independent nerve-trunks, two from each lateral 

 ganglion. The former may occupy any position upon the 

 posterior sides of the ganglions from the dorsal end, which 

 correspond to their position in Fredericella, to the neigh- 

 borhood of the epistomic trunk, while the latter have an 

 equal range on the anterior side. With the exception of 

 the epistomic nerve-trunk there is, at least in the zooids 

 of Pectinatella magnitica, no regularity in the origin of the 

 nerve-trunks within the limits above described, and, besides 

 this, the character of the nerves themselves and the form 

 of the nerve-mass may be greatly altered in different 

 zooids of the same stock. Thus in PL 11, figs. 1, 2, 8', 

 the two dextral ganglions are given from two different 

 zooids of the same colony which reverse each others out- 

 lines, the small ventral end of fig. 2, corresponding to the 

 large ventral end of the ganglion in fig. 1, and the large 

 dorsal end of the former to the small dorsal end of the 

 latter. 



The same figures also show the variability of the 

 nerve-truuks. The epistomic nerve in one springs directly 

 from the end of the ganglion, while in the other it rests on a 

 raised pediment on the posterior side, and the lophophoric 

 nerve-trunk in one comes from the posterior side of the 

 ganglion and is made up of confluent nerves ; in the oth- 



