The sequences in appearance of tentacles on buds are systematically discussed on those species of hydras of which in almost all the known species the sequence has been described. They may be classified in five kinds of tentacular patterns indicated by the species name of a representative of each species group, viz. circumcincta-, magnipapillata-, intaba-, canadensis-, and oligactis-type, plus a species group in which the tentacles arise successively but show no definite pattern. In the circumcincta-type, six tentacles appear almost simultaneously in a radially symmetrical manner, while in the other, four types they arise successively with bilateral symmetry, viz. in pair at a distal-proximal axis of a parent, not in a biradially symmetrical manner, and develop into equal length later on. The bilateral symmetry in the latter four types of the tentacle formation is gradual in its differentiation. Namely, the oligactis-type is considered to be much more stable and distinct in bilaterality than the magnipapillta-type, as known from their tentacle patterns on buds themselves and also from our data on the patterns in regenerating parents of Pelmatohydra robusta and Hydra magnipapillata. The intaba- and the canadensis-type appear to resemble each other as regards the bilaterality, which is in them slightly more distinct than in the magnipapillata-type. In three species groups of the circumcincta-, the magnipapillata-, and the oligactis-type whose tentacular patterns have been known of a considerable number of species, there is seen the correspondence in some important features of taxonomy between members of the groups. There is also found a noticeable agreement between systematic interrelationships among these three species groups and the grade of differentiation in the symmetry from the circumcincta- to the oligactis-type. The Hydrozoa are radially symmetrical in the arrangement of tentacles in their polyps. A few species of them show the bilateral symmetry in the tentacles but these forms also appear to show originally the radial one. The bilaterality in the Hydridae which is an original but not such a secondary product seems to be the only case known of the hydropolyps. It is also interesting that the gradual differentiation and the secondary change in the symmetry in the Hydridae as stated above show some resemblance respectively to the crradual differentiation in arrangement of mesenteries and to the secondary change from bilateral to biradial symmetry in the Anthozoa.
The Oligochaeta are probably to be looked on as descended ultimately from marine ancestor or ancestors. Then, it is a question that the Oligochaeta may be mono-phyletic or poly-phyletic. Considering this problem, the writer has examined his revised system (1966) and concluded that the Oligochaeta seem to be di-phyletic, one ancestor (Fig 2. B) being origin of Archioligochaeta and the other (Fig 2. C) being the origin of Euoligochaeta.