Although the tunicates resemble worms in many points of structure, it is interesting to note that in their young or larval state the body consists of two cavities, one of which contains the internal organs, while in the other the central portion of the nervous system is developed, in which respects they resemble the vertebrate or back-boned animals--fishes, amphibians, reptiles, birds, and mammals. At this stage, too, the creatures possess a tail that is supported by a rod of gristle similar to that which gives place to the backbone in the developing vertebrate. These features, though only transitory, are regarded as a mark of relationship to the higher forms of animal life, and thus the tunicates have been separated from the sub-kingdom Vermes by some zoologists, and given an exalted place at the top of the invertebrate scale, where they form a sub-kingdom of their own, and are looked upon as a link connecting the invertebrates with the vertebrates. [Illustration: FIG. 125.--SEA SQUIRT] Before passing on to the next sub-kingdom, we should observe that the interesting Rotifers or Wheel Animals also belong to the Vermes; but although many of these minute creatures are to be found in sea water, their principal home is the stagnant water of fresh-water ponds and ditches, and thus we may be excused for neglecting them here. CHAPTER XII _MARINE MOLLUSCS_ The sub-kingdom _Mollusca_ includes a great variety of soft-bodied animals which differ from the members of the last division in the fact that they are never segmented, and in the possession of a thick outer covering, of a leathery nature, which completely envelops the body, and which usually secretes a calcareous shell of one or more parts. A general idea of the extent of the group may be formed when we state that it contains the Octopus and the Cuttlefish; all Snails and Slugs, and animals of a similar nature; and all those numerous ‘bivalves’ which are represented by the well-known Oysters, Mussels, Scallops, &c. By far the greater number of the molluscs are aquatic in habit; and of these such a large proportion are marine that the group provides plenty of occupation for the sea-side naturalist. This being the case, we shall devote the present chapter to a description of the general characteristics of these animals, and to the principles of their classification, illustrating our remarks by a few selections from all the chief divisions.