800 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



are homologous throughout the Pisces: in Teleostei, Ganoidei, 

 and Dipnoi 1 , the ribs, placed on the inner face of the body-wall, 

 are serially homologous with the ventral parts of the haemal 

 arches of the tail ; in Elasmobranchii, on the other hand, the ribs 

 are neither serially homologous with the haemal arches of the 

 tail nor homologous with the ribs of Teleostei and Ganoidei, but 

 are outgrowths of the haemal processes into the space between 

 the dorso-lateral and ventro-lateral 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 Elasmobran- 

 chii are provided with homologous haemal arches, which are 

 formed by the coalescence below the caudal vein of simple pro- 

 longations of the primitive haemal processes of the embryo. The 

 canal enclosed by the haemal arches can be demonstrated em- 

 bryologically 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 arrange- 

 ment is probably retained, the ribs are attached to the haemal 

 processes,and are placed immediately without the peritoneal mem- 

 brane at the insertions of the intermuscular septa. These ribs are 

 in many instances (Lepidosteus, Acipenser], and very probably in 

 all, developed continuously with the haemal processes, and be- 

 come 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, Lepidos- 

 teus, 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 



1 We .find the serial homology of the ribs and ventral parts of the haemal arches to 

 be very clear in Ceratodus. Wiedersheim states that it is not clear in Protopterus, 

 although he holds that the facts are in favour of this view. 



