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 Cœlenterata and the Class Hydrozoa.
The second of the great groups or phyla into which the metazoa are divided is the Cœlenterata, 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 cœlenteron 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 cœlenterate 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.
The simplest in structure of the cœlenterates are those that constitute the class Hydrozoa. In this class the primitive central cavity is not divided up by muscular partitions and there is no folding in of the anterior part of the body to form an œsophagus or stomatodæum such as is found in the sea-anemones and coral polyps. In many species and genera the life-history is complex, illustrating what is called the alternation of generations. That is to say, only alternate generations attain sexual maturity, those that do so being produced as buds from a sexless generation, which itself arises from the fertilized eggs of a previous sexual generation. The sexual forms as a rule differ considerably in structure from the sexless ones; many medusæ are the sexual individuals in a life-cycle in which those of the sexless generation are sedentary.