Another class (Trematoda) is made up of parasites called "Flukes," to some of which (e.g., Monostomum) reference will have hereafter to be made with respect to their processes of development.
The class Turbellaria contains a variety of other worms of a lowly kind, one or two of which (e.g., Borlesia) live coiled up in complex tangles which, if unravelled, would attain a length of forty feet. Amongst the commoner kinds may be mentioned the worm Nemertes, and all worms called Planariæ (which are mostly fresh-water, though some live on land), allied to the flukes.
The class of tape-worms (Cestoidea) is one most numerous in its kinds, which are all completely parasitic in habit. Some of them are so fatal in their effects that they are estimated to occasion every seventh death which occurs in Iceland, and they cause mortality amidst our own flocks, producing in sheep the disease known as the "staggers."
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 CŒLENTERA, and a good type of the cœlenterates, 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 cœlenterates 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.
Other cœlenterates, 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.