GENERAL CONSIDERATIONS. 577 



Hatschek (No. 8) has therefore maintained that the velar opening 

 of Amphioxus is not to be regarded as a gill-slit. 



The attempt has often been made, in following out these ideas 

 further, to discover vestiges of the primary vertebrate mouth, which 

 has been sought in the rhomboidal fossa, the pineal gland and the 

 hypophysis. Only recently, Beard (No. 27) and Kupffer (No. 38^ 

 have pointed to the hypophysis as the primary mouth (palaeostoma) 

 of the Vertebi*ata. . 



An altogether satisfactory solution of the difficulties connected with 

 all these questions is, however, at present wanting. 



Attempts have been made in other directions also to find the con- 

 nection between the Vertebrates and the Annelida. We may recall 

 the discovery by Eisig of organs in the Oapitellidae comparable to 

 the lateral line of the Vertebrata, the homologising of the spinal 

 ganglia of the Vertebrates with the parapodial ganglia of the Annelida 

 by Kleinenberg (No. 36), the derivation of the unpaired fin of the 

 Selachians from fused Annelidan parapodia by P. Mayer (No. 39), 

 the attempt to derive the vertebrate eye from the Annelidan eye 

 by v. Kennel (No. 35a), etc. For its support, the Annelidan 

 hypothesis has required a number of sub-hypotheses. Nevertheless, 

 we must admit that the gap which divides the Annelida from the 

 Chordata is even now very considerable, and, as Balfour pointed 

 out, the two most typical organs of the Chordata, the notochord and 

 the gill-clefts, are not foreshadowed in the Annelids. Attempts have 

 not been wanting, it is true, to find the equivalents of these organs 

 in the Annelids. The origin of the mouth from paired rudiments in 

 the buds of Nads and Chaetoyazter (Semper) was compared to the 

 formation of gill-clefts, while the most varied structures in the 

 Annelida were regarded as homologues of the chorda. With regard 

 to this latter point, the view that most deserves attention is that of 

 Ehlers (No. 32) and Eisig (No. 33), who see in the so-called 

 accessory intestine of the Capitfillidae and the Eunicidae (and in 

 similar structures in the Gephyrea) the homologue of the notochord. 

 On the other hand, it should be mentioned that the researches of 

 Kleinenberg (No. 36) on this point did not lead to satisfactory 

 results. " In the development of most Annelids," says Kleinenberg, 

 " there is no trace of the accessory intestine ; I found it only in the 

 larvae of those forms which possess it when adult, viz., the Oapitel- 

 lidae and the Eunicidae. In a larva belonging to the last of these 

 families it hangs as a somewhat short loop beneath the principal 



intestine and opens both anteriorly and posteriorly into the latter. 



PP 



