344. EMBRYOLOGY 



to our point of view, are opposed to a derivation of the Tro- 

 chophore from the medusoid form, and have already made 

 some suggestions respecting a derivation of the Trochophore 

 which, although based upon hypothetical grounds, neverthe- 

 less appear to be better supported by the facts of compara- 

 tive anatomy and embryology than the former view. This 

 view brings the Trochophore into relation with the ancestral 

 forms of the Nemertini, Turhellaria, and Gtenopliora, and 

 regards it as having arisen tolerably directly from much 

 more primitive coelenterate forms than is possible on the 

 assumption of derivation from Medusae. It should be ex- 

 pressly noted here that we necessarily abandon the realm of 

 positive demonstration in making these statements, which 

 scarcely have any higher value than that of mere conjec- 

 tures. 



To as the facts appear to indicate that the ancestral form 

 arose rather directly from a uniformly ciliated gastrula- 

 like archetype by a change in the mode of locomotion. Such 

 a primitive, completely and uniformly ciliated organism 

 exhibited an anterior apical and a posterior oral pole. 

 Secondary axes had not yet been developed ; the form 

 presented at first the monaxial heteropolar type. It is 

 possible, and in view of the ancestors of the Ctenophora 

 probable, that on this form certain differentiations made 

 their appearance without causing an abandonment of the 

 monaxial, heteropolar form, or the radial form that arose 

 from it. Among these differentiations we reckon a tuft of 

 cilia at the animal pole functioning as a rudder (the earliest 

 fundament of the apical plate), an ectodermal pharyngeal 

 tube, and the formation of diverticula of the entodermal 

 portion of the intestine, by the regular distribution of which 

 around the chief axis the first impetus to the formation of 

 definite secondary axes was probably given. 



It must be mentioned that many Actinian larvse present exactly the 

 structure described {Scyphuia). However, this resemblance is probably 

 founded merely on analogy, for in the Cnidaria we assume that the 

 formation of radial gastral pouches took place only after attachment 

 and the development of an Archhydra stage, whereas the Cfcenophora and 

 Bilateria probably never had ^n attached ancestral form. 



