108 MORPHOLOGICAL DEVELOPMENT. 



not take place separation of single completed segments, there 

 takes place separation of groups of segments, which are 

 either sexually mature at the time they drop off or presently 

 become so. And the groups of segments which have become 

 sexually mature before they drop off, have simultaneously 

 acquired swimming organs and developed eyes, enabling them 

 to spread and diffuse the species. Sundry biologists recognize 

 a parallelism between that detachment of developed segments 

 which goes on in the cestoid Entozoa, and that which goes on 

 in the Scypliomedusce. The successively detached members 

 of the strobila are sexually-matured or maturing individuals 

 which, as medusae, are fitted for swimming about, multiply- 

 ing, and reaching other habitats; while each detached pro- 

 glottis of the cestoid is, by the nature of its medium, limited 

 to creeping about. Clearly this fissiparous process in such 

 Annelids as the Syllidce, which has similarly been compared 

 to the stabilization of the Scypliomedusce, differs simply in 

 the respect that single segments are not adapted for locomo- 

 tion, and it therefore profits the species to separate in groups. 

 All these facts and analogies point to the conclusion that the 

 remote ancestor of the Annelids was an unsegmented crea- 

 ture homologous with each of the segments of an existing 

 Annelid. 



This conclusion is supported by other kinds of evidence 

 here to be added. The Iarva3 of Annelids are very various ; 

 but amid their differences there is a recognizable type. " The 

 Trochophore is the typical larval form of the Annelid stem " : 

 a trochophore being a curious spheroidal ciliated structure 

 suggestive of ccelenterate affinities. And this unsegmented 

 larva, representing the remote ancestor from which the many 

 Annelid types diverged, is similar to the larvaa of the Rotifera 

 and the Mollusca : a trochophore is common to all these great 

 classes. Moreover since, among the Rhizota (a sub-class of 

 the Rotiferce), there is a species, Trochosphcera, solitary and 

 free-swimming, resembling in form and structure a trocho- 

 phore, though it is not a larva but an adult, we get further 



