Significant traits of development are exhibited in common by two groups otherwise unallied--certain of the _Platyhelminthes_ and certain of the lower _Annulosa_. Of the _Platyhelminthes_ the ordinary type is an unsegmented creature: a 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 _Caryophyllæus_, 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 continue the race. In both of these types the condition under which the gemmiparously-produced members remain connected, is that they shall be able to feed individually: in the one case by lateral mouths, in the other case by absorption 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 parallelisms. 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 segmentation 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 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 _Scyphomedusæ_. The successively detached members