Those who are familiar with the existing condition of our knowledge of animal morphology will be aware that any such scheme must needs, at present, be tentative and subject to extensive revision, in correspondence with the advance of knowledge. Nor will they regard it as any objection to the scheme of classification proposed, that the divisions sketched out may be incapable of sharp definition--the constant tendency of modern investigations being to break through all boundaries of groups, and to fill up the gaps between them by the discovery of transitional forms. In the place of assemblages of distinctly definable groups, which it has hitherto been the object of the taxonomist to define and co-ordinate in precise logical categories, we are gradually learning to substitute series, in which all the modifications by which a fundamental form passes from lower to higher degrees of organic complication are summed up. ANIMALIA I.--PROTOZOA i. MONERA Protamœbidæ. Protomonadidæ. Myxastridæ, etc. ii. ENDOPLASTICA Amœbidæ. Flagellata. Gregarinidæ. Acinetida. Ciliata. Radiolaria II.--METAZOA A. Gastrææ i. POLYSTOMATA Porifera (or Spongida) ii. MONOSTOMATA 1. ARCHÆOSTOMATA α. _Scolecimorpha_ β. _Cœlenterata_ | /-----------------|----------------\ Rotifera Nematoidea Hirudinea Hydrozoa Turbellaria Oligochæta Actinozoa Trematoda 2. DEUTEROSTOMATA _a._ Schizocœla | /-------------------------|-------------------------\ _Annelida (Polychæta)_ _Gephyrea_ _Brachiopoda (?)_ _Arthropoda_ _Mollusca_ _Polyzoa (?)_ _b._ Enterocœla | /------------------------|------------------------\ _Enteropneusta_ _Chætognatha_ _Echinodermata_ _c._ Epicœla | /------------|----------\ _Tunicata_ _Vertebrata_ B. Agastrææ (provisional) Acanthocephala. Cestoidea THE FOUR CLASSES --BARON CUVIER