Certain minute organisms, familiarly known as "Wheel-Animalcules," or Rotifers, form the "class" _Rotifera_. They have gained their name through an apparently (though, of course, not really) rotary motion, of that end of their bodies at which the mouth is situated. Here also may be mentioned certain curious aquatic worms called _Gasterotricha_, which are closely allied to the wheel animalcules.
Finally may be mentioned the class _Gephyrea_, containing animals, worm-like indeed in form, but which have much apparent affinity to the group next to be spoken of--the group of star-fishes and their allies. Amongst the _Gephyrea_ may be mentioned the worms called _Sipunculus_ and _Priapulus_.
This leads us to the sub-kingdom containing the star-fishes--the sub-kingdom ECHINODERMA, which includes, besides the star-fishes (or _Asteridea_), all sea-eggs or sea-urchins (_Echinidea_), the brittle-stars _Ophiuridea_, as well as the elongated soft animals called sea-cucumbers, or _Holothuridea_, some of which latter are known as the Japanese edible, "Trepang."
Besides these groups there are still surviving a few creatures (_Comatula_ and _Pentacrinus_) belonging to the class of "sea-lilies," or _Crinoidea_, creatures which once lived in countless multitudes, but have now almost entirely passed away. All these crinoids were like star-fishes on stalks, and of the existing forms, _Pentacrinus_ still passes the whole of its life, and _Comatula_ its youth, in a stalked condition.
The next great primary division, or sub-kingdom of animals, is COELENTERA, and a good type of the coelenterates, the sea anemone (_Actinia_), has now become a familiar object to us in our aquaria. These animals are plant-animals, or zoophytes, and some of them build up coral-reefs, or islands, and it is one kind which produces the red coral of commerce. Forms essentially similar, but the solid supporting framework of which is of a softer nature, are such as _Alcyonium_ and _Pennatula_. All these belong to the "class" _Actinozoa_. There are other coelenterates of an active free-swimming habit, such as _Beröe_ and _Cydippe_, which are balls of glassy transparency displaying iridescent hues as they move rapidly through the water by means of their peculiar locomotive organs.