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CHAPTER XII.

CELENTERATA.

CÆLENTERATA (kôiλov, hollow; and evтepov, intestine).—All the members of this sub-kingdom are aquatic, and commonly called "zoophytes" (Sov, an animal; and puróv, a plant), or plant-like animals, in consequence of the striking resemblance which many of them bear to plants, often resembling flowers in their appearance. The body is composed of a soft, sarcode substance, covered externally by a membrane named the " ectoderm," and internally by the "endoderm." In nearly all the ectoderm is furnished with peculiar microscopic organs, termed "cnidæ," "nematocysts," thread-cells," or "nettle-cells," which confer on these animals the urticating or stinging property so well known to all sea-bathers in the case of the common jellyfish. The alimentary canal communicates with the general body cavity, and has only one aperture, which performs the duties of both mouth and anus. There are no distinct organs of respiration and circulation. The nervous system, when present, consists of a solitary ganglion, from which small filaments take their origin. Ocelli and auditory sacs are, for the last time, met with in this sub-kingdom. Reproduction is either sexual, non-sexual (fission or gemmation), or both. They are divided into

1. Hydrozoa (dwp, water; and (a, animals).
2. Actinozoa (aктís, a ray; and Swa).

1. HYDROZOA. These animals have no distinct digestive cavity-in other words, the body cavity and digestive cavity are identical, and the organs of reproduction are situated on the external surface of the body. They are divided into the following orders:—

I. Acalephæ (akaλýøŋ, a nettle—Medusæ), e.g., Jelly-fish or
Sea-nettles.

II. Hydræ (vdwp, water), e.g., Fresh-water Polyps.

III. Sertularidæ (sertula, a plant; and eîdos, form,) e.g., some Sea Firs and their allies.

IV. Campanularidæ (campanula; and eidos), e.g., do., do. V. Tubularidæ (tubus, a tube; and eîdos), Corynidæ.

2. ACTINOZOA. These animals have a distinct digestive cavity that opens into the general body cavity, and is separated from the body walls by the "perivisceral" space. The organs of reproduction are internal. The following

orders are included in this class :

I. Actinæ (aktís, a ray), e.g., "Sea Anemones," "Animal
Flowers."

II. Alcyonidæ (ảλkvóvelov, bastard sponge), e.g., “Dead men's toes," "Cow paps.

III. Madreporidæ, e.g., Tree Corals.

"Sea pens."

IV. Corallidæ (коpάλλιov, coral; and eîdos), e.g., Corals.
V. Pennatulidæ (penna, a pen; and eîdos),
VI. Rugosa (rugosus, shrivelled).—(Extinct).

CHAPTER XIII

PROTOZOA.

PROTOZOA (πрra, first; and (a, animals).—This subkingdom forms the last and lowest division of the animal world, and comprises a vast number of animals, the great majority of which are microscopic, and are commonly termed "animalcules; the sponges, however, form an important exception. They are entirely composed of a simple jellylike substance called sarcode (ráp§, flesh; and eldos, form), and have no distinctive organs whatever-in fact, they are chiefly known by negative characteristics. A distinct alimentary canal and nervous system are totally wanting, hence the members of this sub-kingdom are called acrita (ẻ, not; and κp, I discern); separate organs of respiration, circulation, vision, and audition are also absent. This sarcode, or, as it has been termed, protoplasm, of which they are composed, usually possesses the power of active locomotion and contractility; it sometimes contains only minute granules or molecules, often a nucleus or nucleolus, and frequently small cavities called "contractile vesicles," supposed by some to be rudimentary organs of circulation. They are generally gemmiparous or fissiparous in their reproduction; some, however, multiply by sexual intercourse. They are divided into

1. Stomata (oróμa, the mouth).

2. Astomata (à, not; and σróμa).

1. STOMATA comprise those minute animalcules that are frequently found in organic infusions, and are consequently denominated infusoria. It is still "sub judice" whether

they are developed from pre-existent, undiscovered microscopic germs, or are produced by what has been termed "spontaneous generation" from the materials existing in the organic infusion; be that as it may, they undoubtedly exist and constitute the most highly organised division of the Protozoa. All the infusoria possess a mouth (the acineta has many mouths, hence the name polystome—ñoλú, many ; and σróμa, a mouth-given to this order by Professor Greene), and are furnished with vibratile ciliæ; the mouth does not open into any distinct digestive cavity, but into the soft sarcode of which the animal is composed. Spaces named vacuoles are found in this sarcode, which Ehrenberg believed to be stomachs, hence he named them polygastrica (ñóλλŋ, many; and yaorh3, a stomach). They are divided into

I. Ciliata (cilium), e.g., Epistylis, Paramecium.

II. Suctoria (sugo, I suck), e.g., Acineta.

III. Flagellata (flagellum, a whip), e.g., Peranema.

IV. *Noctiluca (nox, night; and lucco, I shine), e.g., Noctiluca miliaris.

2. ASTOMATA. This division of the Protozoa comprises those animals in which there is no mouth whatever; they are first divided into

1. RHIZOPODA (pía, a root; and woús, the foot).—These animals are characterised by the peculiar property which they possess of throwing out processes of sarcode called pseudo-podia (eudhs, false; and woús). They are divided. into the following orders:

I. SPONGIDA (σñóyyos, a sponge; and eldos, form).—These animals consist of a framework of horny fibres, or of calcareous or flinty processes, covered with the sponge flesh.

* Principally cause the luminosity of the sea.

There are numerous holes leading into the "aquiferous" canals that permeate the sponge in all directions, and through which the water circulates, carrying food, which is taken from it by the pseudo-podia belonging to every individual sponge particle. The great majority of the holes are small, and are called "pores" or "inhalant apertures;' by these the water enters. A few of the holes are large, and are termed "oscula" or "exhalant apertures;" through these the water is ejected. The sponge has been compared, by Professor Huxley, to "a kind of sub-aqueous city where the people are arranged about the streets and roads in such a manner that each can easily appropriate his food from the water as it passes along." This circulation is maintained by cilia waving towards the interior of the sponge.

II. RADIOLARIA (radius, a ray) are small marine animals, composed of a jelly-like sarcode that has the power of throwing out long thread-like pseudo-podia, and of secreting a silicious or flinty skeleton, either in the form of a shell or of separate spicules. This order includes three families, viz. : Acantho-metrina (kavea, a thorn; and μéтpov, measure), Polycystina (6λλn, many; and KúσTIS, a bladder), and Thallasicollida (@axáoσa, sea; and кóλλα, glue).

III. FORAMINIFERA (foramen, an aperture; and ferens, bearing).—The members of this order consist of a simple sarcode substance that has the power of throwing out long thread-like pseudo-podia, which interlace with each other so as to constitute a net-work termed "animated spider's web." The body is always protected by a shell or "test," which is either calcareous, i.e., formed of carbonate of lime, or arenaceous,” i.e., formed of sand, welded together by an animal cement. These shells are either mono-thalamous (μóvos, single; and @áλaμos, a chamber), single-chambered, or polythalamous (róλus, many; and edλauos), many-cham

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