Other coelenterates, of the same essential type but of simpler structure, form the class _Hydrozoa_. Amongst these may be mentioned the little _Hydra_ of our ponds, which will often come before us in our survey of animal life. Some compound forms of Hydrozoa simulate the compound Actinozoa; such are the calcareous millipores, and those with a softer structure, called "corallines," such as _Eudendrium_ and many others. The Portuguese man-of-war (_Physalia_) and the various forms of jelly-fish (_Medusæ_) all belong to the _Hydrozoa_, as also does a very curious and very elementary form, to which the name _Tetraplatia_ has been given.
Next we come to the group of sponges, SPONGIDA, some of which--as the now well-known _Euplectella_--are of marvellous beauty and delicacy of structure; while others, as the sponge of commerce, are of much greater simplicity of form. Simplest of all the sponges is the sponge called _Ascetta Primordialis_. Some sponges have a horny, some a calcareous, and some a siliceous skeleton, and (strange as it may appear) some have a habit of boring into shells, and living in the excavations they make.
An animal recently discovered, _Dicyema_, may at this initial stage of our inquiry be left with its place and affinities undetermined. It is a minute worm-like creature of most exceptionally simple structure, which lives parasitically within cuttle-fishes.
We now pass to animals (if so they are really to be considered) which are the lowest and simplest of all, and which are mostly microscopic in size, and may be grouped together under the term HYPOZOA, or under the generally employed name _Protozoa_. With very few exceptions these animals are aquatic, and if terrestrial they are found in damp localities. Some are marine, others are fresh-water organisms.
The highest of the group are the animalcules, which are named _Infusoria_, most of which are freely swimming organisms, though a certain number of them live fixed to some supporting body.
Another group of _Hypozoa_ is that termed _Gregarinida_, a group made up of very lowly parasites, such as are often found tenanting the intestines of insects as well as those of higher animals. Finally, we have the group of _Rhizopoda_, animals which have the faculty of projecting and retracting (so to say, at will) filamentary or conical processes of their semi-fluid substance, such processes being the _Pseudopodia_, which were referred to earlier.[17]