254 M. Edmond Perrier on the 



to mortify, and renders it unsuited to the development of 

 vibratile cilia. This pro|)erty manifests itself in these animals 

 almost at the very beginning of the development of the 

 embryo [Nanplius) ; it has rendered necessary the ecdyses 

 which have in their turn occasioned the metamorphoses ; the 

 absence of the cilia has had to be compensated for by the 

 formation of jointed legs, moved by striped muscles and 

 liavino- the respiratory apparatus dependent upon them. 

 Commencing at any rate from the precocious period which 

 the A'aznj/^/s represents in their ontogeny, the evolution of the 

 Arthropods has therefore taken quite a peculiar direction, and 

 they have remained isolated from all animals the epithelia of 

 which have continued to be wholly or partially ciliated. \f the 

 law of patrogony is correct, which no one disputes, there 

 cannot exist beyond the Rotifers (Scirtopoda) a transitional 

 form between them and the long uninterrupted series of the 

 Nephridiates which proceeds from the Rotifers to the Verte- 

 brates inclusively. Tiiis excludes them from tlie lineage of 

 the Vertebrates, the apparent resemblances between which 

 and the Merostomata, the Arachnids, or the Crustaceans are 

 only cases of convergence. In particular the protective 

 shields of the placoid tishes are actual bones formed in the 

 derm, and not a simple epidermic covering like the pieces of 

 the Arthropod carapace. 



(3) Branchial Clefts. — The presence of lateral branchial 

 clefts in Balanoglossus and in the Appendicularidas has been 

 one of the great arguments that have been invoked in favour 

 of their relationship with the Vertebrates, This relationship 

 is undeniable in the case of the Appendicularidte, but the 

 absence of embryonic segmentation in these animals proves 

 (law of patrogony), as we have already pointed out, that they 

 are not ancestral Vertebroids, but degraded Vertebroids ; the 

 same remark would apply to Balanoglossus , in case we were 

 to admit as real the resemblances which people have striven 

 to find between it and the Vertebrates, apart from its branchial 

 clefts. But this latter resemblance is itself open to suspicion. 

 The branchial clefts of the Vertebrates and the primary and 

 secondary branchial clefts of Amphioxus are repeated, in fact, 

 exactly like the metaraeres; although the embryo of Balano- 

 glossus exhibits manifest traces of metameric segmentation, 

 there is no relation between the metameres and the branchial 

 clefts. This would be explained, for once in a way, as is 

 shown by the discord which iinally supervenes in the case of 

 Aviphiocuus itself, under the supposition of a degeneration on 

 the part of Balanoglossus^ but not on the hypothesis which 

 makes it an anccatral form. The absence of branchial clelts 



