of the Animal Kingdom. 331 
number of Arthropoda breathing by trachez has lately, since 
the structure and development of Per‘patus have been more 
accurately known, been increased by a new class, namely that 
of the Protracheata (of Balfour and others). Since Moseley 
succeeded in proving that these animals, which for a long 
time were referred to the Vermes under the name of Onycho- 
phora, really respire by trachese, and were also distinguished 
from the other Arthropoda by the possession of quite diver- 
gent, primitively constructed, and foot-like buccal organs *, 
I regard it as perfectly right to unite this small but interesting 
group, after the example of recent naturalists, with the stem 
of the Arthropoda as a separate branch of the Tracheata. 
In like manner I have regarded it as more judicious and 
more in accordance with nature to establish in the class Crus- 
tacea, besides the chief groups hitherto adopted (Thoraco- 
straca, Arthrostraca, and Entomostraca), a fourth group, 
namely that of the Paleostraca. This section of the Crus- 
tacea, which includes the Trilobita and the Xiphosura, as 
well as the fossil Merostomata first described by H. Wood- 
ward, is distinguished from all others by the fact that the 
mouth-organs of the forms belonging to it are very different 
from those of the other Crustacea, and either foot-like or rudi- 
mentary, or sometimes even entirely wanting (Trilobita). 
Finally, as regards the Chordo- Vertebrata, I find myself 
led in some points to adopt views somewhat diverging from 
those of other naturalists. Thus I agree perfectly with 
Balfour, Ray Lankester, &c., that we must unite the Uro- 
chorda (Tunicata) as well as the Cephalochorda (Leptocardii) 
with the stem of the Vertebrata; but with respect to the 
Cyclostomi or Monorhina I by no means share the opinion of 
those savants. ‘The Cyclostomi are in fact distinguished by 
a great number of partly positive, partly negative, characters 
of importance, which never occur elsewhere in other fishes 
(Selachii, Ganoidei, Dipnoi, and Teleostei), or, indeed, in any 
true Vertebrata even during the embryonal period; and by 
these peculiarities they evidently, on the other hand, nearly 
approach the Leptocardii and Tunicata. These characters 
are :— 
a. The absence of the mandible. 
b. The absence of paired extremities. 
c. The absence of the nervus sympathicus. 
d. The originally simple and unpaired nasal cavity 
(olfactory cavity) with a simple external dorsal 
olfactory aperture, 
* Phil. Trans. vol. clxiv. (1874). 
22* 
