404 
MESSRS. F. M. BALFOUR AND W. N. PARKER OK THE 
muscles, which may perhaps have their homologues in Teleostei and Ganoids in certain 
accessory processes of the vertebrae. 
The other view, which we are inclined to adopt, and the arguments for which have 
been stated in the preceding pages, is as follows :—The Teleostei, Ganoidei, Dipnoi, 
and Elasmobranchii are provided with homologous haemal arches, which are formed by 
the coalescence below the caudal vein of simple prolongations of the primitive haemal 
processes of the embryo. The canal enclosed by the haemal arches can be demon¬ 
strated embryologically to be the aborted body cavity. 
In the region of the trunk the haemal processes and their prolongations behave 
somewhat differently in the different types. In Ganoids and Dipnoi, in which the 
most primitive arrangement is probably retained, the ribs are attached to the haemal 
processes, and are placed immediately without the peritoneal membrane at the inser¬ 
tions of the intermuscular septa. These ribs are in many instances {Lepidosteus, 
Acipenser), and very probably in all, developed continuously with the haemal processes, 
and become subsequently segmented from them. They are serially homologous with 
the ventral parts of the haemal arches of the tail, which, like them, are in many 
instances ( Ceratodus „ Lepidosteus, Polypterus, and to some extent in Amia) segmented 
off from the basal parts of the haemal arches. 
In Teleostei the ribs have the same position and relations as those in Ganoids and 
Dipnoi, but their serial homology with the ventral parts of the haemal processes of the 
tail, is often ( e.g ., the Salmon) obscured by some of the anterior haemal arches in the 
posterior part of the trunk being completed, not by the ribs, but by independent 
outgrowths of the basal parts of the haemal processes. 
In Elasmobranchii a still further divergence from the primitive arrangement is 
present. The ribs appear to have passed outwards along the intermuscular septa into 
the muscles, and are placed between the dorso-lateral and ventro-lateral muscles (a 
change of position of the ribs of the same nature, but affecting only their ends, is 
observable in Lepidosteus), This change of position, combined probably with the 
secondary formation of a certain number of anterior haemal arches similar to those 
in the Salmon, renders their serial homology with the ventral parts of the haemal 
processes of the tail far less clear than in other types, and further proof is required 
before such homology can be considered as definitely established. 
This is not the place to enter into the obscure question as to how far the ribs of the 
Amphibia and Amniota are homologous with those of Fishes. It is to be remarked, 
however, that the ribs of the Urodela (1) occupy the same position in relation to the 
muscles as the Elasmobranch ribs, (2) that they are connected with the neural arches, 
and (3) that they coexist in the tail with the haemal arches, and seem, therefore, to be 
as different as possible from the ribs of the Dipnoi. 
