In the lake _C. lapidosa_ was accompanied by the gemmules of _Spongilla bombayensis_, but it is interesting that whereas the latter sponge was entirely in a resting condition, the former was in full vegetative vigour, a fact which proves, if proof were necessary, that the similar conditions of environment do not invariably have the same effect on different species of Spongillidæ. APPENDIX TO PART I. FORM OF UNCERTAIN POSITION. (Plate I, fig. 4.) On more than one occasion I have found in my aquarium in Calcutta small sponges of a peculiar type which I am unable to refer with certainty to any of the species described above. Fig. 4, pl. I, represents one of these sponges. They are never more than about a quarter of an inch in diameter and never possess more than one osculum. They are cushion-shaped, colourless and soft. The skeleton-spicules are smooth, sharply pointed, moderately slender and relatively large. They are arranged in definite vertical groups, which project through the dermal membrane, and in irregular transverse formation. Small spherical gemmules are present but have only a thin chitinous covering without spicules or foramen. These sponges probably represent an abnormal form of some well-known species, possibly of _Spongilla carteri_. I have seen nothing like them in natural conditions. PART II. FRESHWATER POLYPS (HYDRIDA). INTRODUCTION TO PART II. I. THE PHYLUM COELENTERATA AND THE CLASS HYDROZOA. The second of the great groups or phyla into which the metazoa are divided is the Coelenterata, in which are included most of the animals commonly known as zoophytes, and also the corals, sea-anemones and jelly-fish. These animals are distinguished from the sponges on the one hand and from the worms, molluscs, arthropods, vertebrates, etc., on the other by possessing a central cavity (the coelenteron or "hollow inside") the walls of which are the walls of the body and consist of _two_ layers of cells separated by a structureless, or apparently structureless, jelly. This cavity has as a main function that of a digestive cavity. An ideally simple coelenterate would not differ much in general appearance from an olynthus (p. 27), but it would have no pores in the body-wall and its upper orifice would probably be surrounded by prolongations of the body-wall in the form of tentacles. There would be no collar-cells, and the cells of the body generally would have a much more fixed and definite position and more regular functions than those of any sponge. The most characteristic of them would be the so-called cnidoblasts. Each of these cells contains a capsule[AK] from which a long thread-like body can be suddenly uncoiled and shot out.