To attempt the reformation of any class in the animal kingdom,― the numerous individuals of which are widely spread over the surface of the globe, many therefore difficult of access, and others, though easily obtained, yet extremely perishable, and for the most part so minute, as to require for their examination the utmost penetration of the microscope and unwearying perseverance in the observer―is a task of no little difficulty int he accomplishment, and one that may fairly entitle him who enters upon it to expect to meet with indulgence. It is probably owing to these retarding circumstances that the class Polypi, as now generally understood, presents such a heterogeneous accumulation of widely differing structures as is perhaps to be found in few similar portions of the animal kingdom: and it is only by a strict investigation of the intimate structure of the various forms of animals that have been so indiscriminately heaped together, that any permanent arrangement that shall indicate their true and natural affinities may be hoped for.