1894- NOTES AND COMMENTS. 245 



their minute structure. Moreover, the explorations of Herr Simonson 

 in the Upper Silurian Limestone of the Island of Oesel, in the Baltic, 

 have recently furnished most of the principal European museums with 

 good specimens. The fossils are thus within reach of most workers. 

 The greatest sensation of late, in connection with this subject, how- 

 ever, is the announcement by Mr. William Psitten, in the Anatomischer 

 Anzeigev, that certain parts of the shield of the existing king-crab, 

 Limnliis, are almost identical in intimate structure with a certain 

 layer in the shield of the Devonian Ptevaspis. We have not yet had 

 the opportunity of verifying the observation ; but if the facts can be 

 established Palaeontology will, after all, furnish some support to the 

 idea that there is a close connection between the primitive Chordata 

 and the so-called Arachnids of the Limulus tribe. We prefer, how- 

 ever, at present to leave the question of the ancestry of the Chordata 

 to the Embryologists, and we cannot do better than allude next to 

 recent researches in this direction. 



The Ancestry of the Chordata. 



The researches of the past ten years have shown a marked ten- 

 dency towards a closer and closer rapprochement between those three 

 groups of enteroccelous animals, the Echinoderma, the Entero- 

 pneusta, and the Chordata. The idea of an affinity between the two 

 first groups dates from the time when Agassiz in America and 

 Metschnikoff in Europe discovered that Tornaria, which had been 

 regarded by Johannes Miiller as a starfish larva, was in reality the 

 larva of the worm-like Balanoglossus. Metschnikoff even went so far 

 as to homologise the proboscis of the adult Balanoglossus with an 

 ambulacral tentacle, and the proboscis-vesicle he regarded not simply 

 as a rudimentary, but as an actually vestigial representative of the 

 water-vascular system of the Echinoderm. It is chiefly to Mr. W. 

 Bateson, on the other hand, that we owe the establishment of the 

 Chordate affinities of the same remarkable animal ; while the whole 

 progress of Echinoderm research, in the able hands of Ludwig, 

 Semon, Bury, and others, has revealed a continually-increasing series 

 of fundamental resemblances between the Echinoderm and the 

 Chordate types. The idea of a real affinity between the three groups 

 has taken deep root, especially in England, and the somewhat ill- 

 founded opposition of Spengel to the view of the Chordate relations of 

 Balanoglossus has effected little beyond the strengthening of the 

 position of those who dissent from his conclusions. 



Mr. Garstang expounded a theory of the inter-relationships of 

 these groups, which he takes to present a simple and consistent 

 explanation of the structure and development of Balanoglossus and the 

 lower Chordata. 



Recent research has shown that the larvae of Echinoderms are 

 all derivable from one common type, which can be imagined as a 



