of the Animal Kingdom. 333 
I think, therefore, that the relationships between the diffe- 
rent classes of the Chordo-Vertebrata may be expressed by a 
more natural grouping, if I divide their main stem into two 
divergent branches or subphyla, one of which, the subphylum 
of the Provertebrata, or .Primitive Vertebrata, includes the 
more or less reduced or retrogressive forms of simpler struc- 
ture, furnished with a wide stomadeum, a hypopharyngeal 
groove, and an unpaired median olfactory organ, but destitute 
of a mandible and of paired limbs, as well as of a nervus 
sympathicus and of true lateral organs, to which belong, as 
distinct classes, the Tunicata, Leptocardii, and Monorhina. 
The other, larger branch, the subphylum of the Metavertebrata, 
or true Vertebrates, would then embrace the true fishes or 
Ichthyozoa (Selachii, Ganoidei, Dipnoi, and Teleostei), ori- 
ginating by progressive phylogenesis the Amphibia, Reptilia, 
Birds, and Mammalia, all of which are furnished with man- 
dibles, paired extremities, paired nasal cavities, and a nervus 
sympathicus (see Table, p. 334). 
As regards the forms of the Provertebrata living in the 
present geological period, ¢¢ appears very probable that they 
are the descendants of the more perfect free primitive Verte- 
brates which existed in Paleozoic times, even before the first 
appearance of the Selachti, of which, however, a great part has 
gradually died out and disappeared entirely in consequence of 
the occurrence of unfavourable conditions of existence in course 
of time. 
And if we consider, in conclusion, that the still existing 
forms of the Provertebrata are for the most part characterized 
by a semi-parasitic or sedentary mode of life, it may be as- 
sumed with some probability that they are indebted for their 
preservation to this their peculiar mode of life. Thus, while 
some of their formerly free-living ancestors had the good for- 
tune to become transformed by natural selection in the way of 
progressive phylogenesis into Metavertebrata, or true Verte- 
brates, and others, on the contrary, in course of time have 
gradually died out, the rest were able, in consequence of 
retrogressive metamorphosis, to maintain their existence, 
although with a reduced structure of body, under the allotted 
conditions of existence, uninjured to the present day, in the 
form of Tunicata, Leptocardii, or Monorhina. 
