_Ascidioida_ _Polyzoa_ / \ / PROTOZOA / _Spongida_ _Infusoria_ _Hydrozoa_ / / COELENTERATA _Actinozoa_ § 100a. Two motives have prompted me to include in its original form the foregoing sketch: the one being that in conformity with the course previously pursued, of giving the successive forms of classifications, it seems desirable to give this form which was approved thirty-odd years ago; and the other being that the explanatory comments remain now as applicable as they were then. Replacement of the diagram by one expressing the relations of classes as they are now conceived, is by no means an easy task; for the conceptions formed of them are unsettled. Concerning the present attitude of zoologists, Prof. MacBride writes:-- "They all recognize a certain number of phyla. Each phylum includes a group of animals about whose relation to each other no one entertains a doubt. Each zoologist, however, has his own idea as to the relationship which the various phyla bear to each other. "The phyla recognized at present are:-- (1) Protozoa. (2) Porifera (Sponges). (3) Coelenterata. (4) Echinodermata. { Cestodes. (5) Platyhelminthes { Trematodes. { Turbellaria. (6) Nemertea. (7) Nematoda. (8) Acanthocephala (Echinorhyncus). (9) Chætognatha (Sagitta). (10) Rotifera. (11) Annelida (Includes Leeches and Gephyrea, Chætifera). (12) Gephyrea, Achæta. { Tracheata (Peripatus, Myriapods, Insects). (13) Arthropods { Arachnids. { Crustacea. { Pycnogonida. (14) Mollusca. (15) Polyzoa (Including Phoronis). (16) Brachiopoda. (17) Chordata (Includes Balanoglossus and Tunicates. Some continental zoologists do not admit Balanoglossus)." [This last phylum of course includes the _Vertebrata_.] Though under present conditions, as above implied, it would be absurd to attempt a definite scheme of relationships, yet it has seemed to me that the adumbration of a scheme, presenting in a vague way such relationships as are generally agreed upon and leaving others indeterminate, may be ventured; and that a general impression hence resulting may be useful. On the adjacent page I have tried to make a tentative arrangement of this kind.