600 The Vertebrates and Invertebrates. [ October, 
ment, that the latter gained the predominance, and that finally 
the old mouth became aborted. 
The next problem attacked is the origin of the gill-clefts. A 
very elaborate account is given of the supposed process by which 
the external gills and segmental organs of Annelids were meta- 
morphosed into the gills and gill-clefts of vertebrates and the 
skeletal elements connected with them. The great difficulty — 
which Dr. Dohrn confesses in this matter is the connection of 
the inner extremities of the segmental organs with the wall of 
_the alimentary canal. But if this be granted it is comparatively 
easy to understand how the shortening and widening of the seg- 
mental organs might give rise to gill-cavities such as those of the 
Selachians. The process by which Dr. Dohrn conceives that the 
limbs of vertebrata might have been developed from two pairs of 
gills in Annelids is a greater evidence of ingenuity, though it is 
to be expected that it will be viewed rather incredulously. 
It follows from the view of the origin of vertebrates thus ex- 
pounded that Amphioxus loses much of its interest, for there is 
no place for Amphioxus among Annelids, nor among the primor- 
dial vertebrates; it lacks almost all that they possess. Yet 
nothing can be gained by excluding Amphioxus from the verte- 
brates ; for it is so connected with the cyclostome fishes that 1t 
cannot be placed at any great distance from them ; while on the 
other hand it is so related to Ascidians that the latter must be 
included among the vertebrata. 
Dr. Dohrn then proceeds with a long argument to show that 
the cyclostome fishes are degenerate from a higher type of fishes, 
and that Amphioxus is a result of still further degeneration. He 
shows how their mode of life necessitates many of the modifica- 
tions they have undergone, and that the diversities of the details 
of structure in eyclostomes are inconsistent with, their being 
viewed as representing stages in upward development. Finally, 
the larva of Ascidians is represented as a degenerate fish — a de- 
generate cyclostome, possibly — which carries to the extreme all 
the departures of the latter from the fish-type. The most 1M- 
portant element in this degeneration results from the fact that 
Ascidians, instead of being attached to fishes or to any object 
from which they can derive nutriment, are fixed to stones, acer 
ete., or to such parts of animals (cephalo-thorax of crabs, tubes 
tubicolous annelids) as do not afford them nourishment. Conse- 
quently they have lost the old mouth in the organ of attachment 
homologous with that of all vertebrates, and have developed @ 
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