Huxley on Lacinularia socialis. 
17 
and there is one very characteristic circumstance, the presence 
of the water-vascular system, in which they agree with them. 
Now, with what Annuloida are the Rotifera most closely 
allied? To deterniine this point, we must ascertain what is 
the fundamental type of organization of the Rotifera. 
Suppose in Lacinularia a line to be drawn from the mouth 
to the anus, and that this be considered as the axis of the body ; 
suppose, again, that the side on which the ganglion lies is the 
dorsal side, the opposite being the ventral ; suppose, also, the 
mouth end to be anterior, the anal end posterior, — then it will 
be found that the lower circlet of cilia upon the trochal disc 
encircles the axis of the body, while the upper circlet of large 
cilia does not encircle the axis, but lies in the lower and an- 
terior region of the body. 
If the region behind that ciliary circlet which is traversed by 
the axis be called the post-trochal region, and that in front of 
it the pre-trochal region, we find that the circlet of large cilia 
is developed in the inferior pre-trochal region. 
Now compare this Rotifer with the larva of an Annelid. 
It will be immediately seen that the two are of essentially 
the same type, only that, while the Annelid larva is equally 
and symmetrically developed in all its regions, and has 
frequently no accessory ciliated bands, the Rotifer has its 
superior post-trochal and inferior pre-trochal regions de- 
veloped in excess ; so that the anus is thrown to the ventral, 
while the mouth is thrust towards the dorsal surface,* an 
accessory ciliated circlet being at the same time developed in 
the latter region. 
Melicerta ringens (compare figs. 26-28) resembles Lacinu- 
laria in the arrangement of its ciliated bands, only they are 
far more distorted from their normal circular form. Tubi- 
colaria closely resembles Melicerta, and there can be little 
doubt that Megalotrocha and Limnias are to be added to this 
division. 
In Brachionus, Philodina, Rotifer, Notommata, the same 
fundamental type obtains, but the deviation from symmetry 
takes place in a different way. 
In all these it is the ventral post-trochal region which is 
over-developed, and therefore the anus is thrown to the dorsal 
or ganglionic side. 
In Notommata the trocha appears to be simple and un- 
altered in most species, and there is no accessory circlet. 
* This over-development is not a mere matter of hy^wthesis. The 
young Lacinularia has the anus nearly terminal, and the " peduncle " only 
subsequently attains its full proportions. Compare fig. 17 and fig. 18, 
pi. L 
VOL. I. C 
