We may next pass to a small sub-kingdom which includes the curious and inert animals before referred to[16] as "Sea-squirts," Tunicaries or Ascidians, and which constitute the sub-kingdom TUNICATA. These are marine organisms of very simple but very peculiar structure which sometimes grow up in compound aggregations. Certain forms (_e.g._, _Pyrosoma_) are luminous at night and may be seen swimming about in the ocean like so many red-hot urn-heaters. As we shall hereafter see, the reproductive processes and the earlier stages of existence of these creatures possess much interest, and have afforded strong grounds for regarding them, in spite of their lowly organization, as very close allies of the highest animals or _Vertebrata_.
Returning now to the "lobster" (lately mentioned as one of those animals commonly called "shell-fish") we may regard it as an example of what is by far the most numerous of all the sub-kingdoms of animals. This sub-kingdom is made up of animals with jointed feet or "Arthropods," and the ARTHROPODA are subdivided into four classes--1, _Crustacea_; 2, _Myriapoda_; 3, _Arachnida_; and 4, _Insecta_; and it is to the first of these four classes that the lobster belongs.
The class _Crustacea_ contains, besides the lobster (and its near allies, hermit-crabs, prawns, shrimps, and cray-fish), all crabs, including those very quaint-looking animals (now so often seen in our living collections), the king-crabs (_Limulus_), and a variety of more or less strangely different forms such as the following:--
Certain Crustaceans, of the group called _Ostracods_, have the hard outer coat of their body so peculiarly modified that they have quite the appearance of Lamellibranch Mollusks, and this resemblance is even more than skin deep, as we shall see later.
Some of another group, called _Copepoda_, become, when adult, so degraded in structure as to have the appearance of mere worms, as _Lerneocera_ and _Tracheliastes_, and become strangely unlike the typical forms (crabs and lobsters) of their class.
Other animals of the class _Crustacea_, which animals form the order _Cirripedia_ (barnacles and acorn-shells), bear such an external resemblance to mollusks that they were actually classed by Cuvier in the class _Mollusca_. In some of them--the Barnacles which commonly attach themselves to the bottoms of ships--the head grows from above downwards to a relatively enormous degree, forming the long stalk or "peduncle," at the lower end of which the small body with its limbs hangs suspended.