94 Evolution and Distribution of Fishes 



tary canal becomes surrounded by and hung in mesoblastic 

 folds, while the muscles and other mesoblastic tissues 

 originate from the anterior three. So Burger says: "in 

 einem Embryo, der einige Tage alt ist, verwachsen endlich 

 der nunmehr unpaare ventrale und dorsale Mesodermsack, 

 so dass nunmehr . . . zwei Zell-blatter existiren, von denen 

 das eine den Darm umgiebt, das andere der Korperwand 

 angepresst ist. Der Spalt zwischen ihnen ist grossen- 

 theils aus Mangel an Raum unterdriickt, kommt indess im 

 hinteren Drittel des Embryos in Gestalt einer ziemlich 

 geraumigen Hohle zur Ausbildung. JVir haben in den beiden 

 Zellbldttern Splanchnopleiira und Somatopleura, in der 

 Hohle ein Coelom vor tins." 



The five plates in Amphioxiis behave much as in 

 nemerteans, but their gradual growth and changes have 

 been more carefully studied by several observers, to whose 

 results the reader is referred. 



Reviewing all of the above comparative details, the 

 writer would strongly emphasize that the metanemerteans 

 show so many structural, functional, and embryological 

 characters which are either common from them to Amphi- 

 oxus and to Petromyzon or even to the higher fishes, or 

 they present so many characters which gradually lead up 

 from them to the three groups named, that they stand out 

 preeminently alongside all other invertebrates as the fore- 

 runners and phylogenetic ancestors of Amphioxiis, the 

 cyclostomes, and the true fishes. 



But now it must be at once and freely acknowledged 

 that remains of nemerteans and even of cyclostomes are 

 unknown palaeontologically. The only doubtful represen- 

 tatives are the "conodonts" of Silurian and other rocks, 

 which are discussed in another chapter (pp. 107-09). But 

 this fact need in no way excite surprise, nor should it cause 

 one to reject the view that both groups may have existed 

 abundantly in Silurian, in Ordovician, or even — for nemer- 

 teans at least — in Cambrian times. For we should scarcely 

 have known of the abundant existence of many elasmo- 

 branch genera from the Devonian period onward till now, 

 had they not evolved the teeth, scales, and plates that occur 

 in Isolated fragments and abundant quantity in not a few 



