464 T. H. MORGAN. 



tition. la the Annelids, for instance, where there is a typical 

 coelom developed, there are no traces of gut pouches, and 

 this lias to be interpreted as a secondary loss. Hatschek has 

 suggestively remarked (15), '^ The two mesodermal teloblasts 

 may correspond to the coelom-sacs from which they were 

 derived by a reduction in the number of their cells. In fact, 

 we find this method of formation only in those cases where the 

 number of cells of the embryo is very small." 



Whatever be the conclusion reached as to the formation of 

 the coelom, we shall still be far from a decision as to how and 

 when the coelom repeated itself in the metameric forms. 



The trochosphere of the Annelids, Mollusca, and Mollus- 

 coidea (?) has been utilised by Hatschek to support the whole 

 family tree of the higher Metazoa (the Vertebrates perhaps 

 excepted). 



Kleinenberg has interpreted the trochosphere as a recapitula- 

 tion of a hydro-medusa. E. B. Wilson and others have rejected 

 the trochosphere ancestry, and believe the larva to be coeno- 

 genic. Whitman wrote in 1887, " In spite of volumes 

 devoted to the discussion of the subject, the larva of Poly- 

 gordius still remains a morphological puzzle." 



The trochosphere theory got a strong support from Semper's 

 discovery of Trochosphsera sequatorialis.^ Even Kor- 

 schelt and Heider, who, as a rule, are most circumspect in 

 accepting embryological speculation, M'rote in 1890, '' Hochst 

 wahrscheinlich liegt in der Trochophora der Anneliden die 

 ontogenetische Recapitulation einer Stammform vor, welche 

 den Anneliden Mollusken und MoUuscoiden gemeinsam war 

 und von der aus sich diese Thierstamme als selbststiindige 

 Gruppen abzweigten." 



Recognising the relationship existing between the trocho- 

 sphere larvae of Mollusca and Annelida, embryologists have not 

 been satisfied to postulate only the archaic nature of the larva, 



* Semper's Rotifer was known long before llatscliek's theory, and sug- 

 gested to me the name " trochosphere " for the larval form (' Devel. of 

 Lynnaius,' 1875), wliich some years later Hatschek adopted from me with 

 the change of tlie word to " trocliophorc." — K. Ray Lankester. 



