THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 107 



Planarian or a Trematode exemplifying it. Among the free 

 forms, as in some Planarians, there occurs transverse fission, 

 and prompt separation of the segments; while among some 

 other free forms, as the Microstomida, the two segments first 

 produced, themselves become segmented while still adherent, 

 and this process is repeated until a string is formed. Another 

 group of the Platyhelminthes, the Cestoid Entozoa, exhibit 

 analogous processes. There are unsegmented forms, as the 

 Caryopliyllaeus, and there are forms in which the segments, 

 now few now many, adhere together in chains; the terminal 

 members of which, however, eventually separate, and having 

 before separation approached the trematode structure, become 

 independent individuals which grow, creep about, and con- 

 tinue the race. In both of these types the condition under 

 which the gemmiparously-produced members remain con- 

 nected, is that they shall be able to feed individually: in 

 the one case by lateral mouths, in the other case by absorp- 

 tion through the integument. It is further observable that 

 in both cases separation of the component individuals occurs 

 at sexual maturity, when advantage in nutrition has ceased 

 to be the dominant need and dispersion of the species has 

 taken its place in degree of importance. Among 



Annelids, higher though they are in type, we find parallel- 

 isms. Usually in its first stage an annelid is unsegmented, 

 but as fast as it elongates lines of segmentation indent its 

 surface. This segmentation proceeds in various ways, and 

 the segments exhibit various degrees of dependence. In 

 some low types, spontaneous fission goes on to the extent of 

 producing single segments, each of which has such vitality 

 that it buds out anterior and posterior parts at its two ends. 

 Thus alike in the simple form which exists before segmenta- 

 tion and in the form exhibited by a detached segment, we 

 have a unit analogous to each of the units which are joined 

 together in certain free Turbellaria and in the Cestoids: the 

 difference being that in the Annelids the sexually-mature 

 units do not individually disunite. But though there does 



