PEDIGREE OF ZOOPHYTES

		Ctenophora					Hydromedusæ
		Tæniata	Lobata				Rhizostomeæ
		│ │	│ │				│ │
		Saccata Stenostoma					Semæostomeæ Discomedusæ
		│		Trachymedusæ			│	Siphonophora
		│		│			│	│
		Eurystoma		│ │	Lucernaria Calycozoa		│ │	│ │
		│ │		│ │	│ │		│ │	│ │
		│		│	│		│	│

		Leptomedusæ
				Coralla		│
				Hexacoralla	Octocoralla	│
				│	│	│
						│
				Tetracoralla		│
Spongiæ					│	│
Fibrospongiæ			Calcispongiæ		│	│
Chalynthina			Leucones	Sycones	│	│
Hexactinella	│	Geodina	Dyssycus	Sycurus	│	Hydroida
│	│	│	│	│	│	│	Cordylophora
│	│	│	│	│	│	│	│	Hydra
					│	│	│	│
│			Ascones		│	│	│	│
│ │	Myxospongia Halisarcina		│ │		│ │	Hydroida
Chalynthus	│		Olynthus		│	│
│	│		│		│	│
Archispongiæ					Hydroida
	│			│
	│			Archydra
	│			│
Protascus
│
Gastræa

The second class of Sea-nettles is formed by the Hood-jellies (Medusæ) or Polyp-jellies (Hydromedusæ). While most corals form stocks like plants, and are attached to the bottom of the sea, the Hood-jellies generally swim about freely in the form of gelatinous bells. There are, however, numbers of them, especially the lower forms, which adhere to the bottom of the sea, and resemble pretty little trees. The lowest and simplest members of this class are the little fresh-water polyps (Hydra and Cordylophora). We may look upon them as but little changed descendants of those Primæval polyps (Archydræ), from which, during the primordial period, the whole division of the Sea-nettles originated. Scarcely distinguishable from the Hydra are the adherent Hydroid polyps (Campanularia, Tubularia), which produce freely swimming medusæ by budding, and out of the eggs of these there again arise adherent polyps. These freely swimming Hood-jellies are mostly of the form of a mushroom, or of an umbrella, from the rim of which many long and delicate tentacles hang. They are among the most beautiful and most interesting inhabitants of the sea. The remarkable history of their lives, and especially the complicated alternation of generation of polyps and medusæ, are among the strongest proofs of the truth of the theory of descent. For just as Medusæ still daily arise out of the Hydroids, did the freely swimming medusa-form originally proceed, phylogenetically, out of the adherent polyp-form. Equally important for the theory of descent is the remarkable division of labour of the individuals, which among some of them is developed to an astonishingly high degree, more especially in the splendid Siphonophora.[(37)] (Plate [VII]. Fig. 13).

The third class of Sea-nettles—the peculiar division of Comb-jellies (Ctenophora), probably developed out of a branch of the Hood-jellies. The Ctenophora, which are also called Ribbed-jellies, possess a body of the form of a cucumber, which, like the body of most Hood-jellies, is as clear and transparent as crystal or cut glass. Comb or Ribbed-jellies are characterized by their peculiar organs of motion, namely, by eight rows of paddling, ciliated leaflets, which run in the form of eight ribs from one end of the longitudinal axis (from the mouth) to the opposite end. Those with narrow mouths (Stenostoma) probably developed later out of those with wide mouths (Eurystoma). (Compare Plate [VII]. Fig. 16.)

The third tribe of the animal kingdom, the phylum of Worms or worm-like animals (Vermes, or Helminthes), contains a number of diverging branches. Some of these numerous branches have developed into well-marked and perfectly independent classes of Worms, but others changed long since into the original, radical forms of the four higher tribes of animals. Each of these four higher tribes (and likewise the tribe of Zoophytes) we may picture to ourselves in the form of a lofty tree, whose branches represent the different classes, orders, families, etc. The phylum of Worms, on the other hand, we have to conceive as a low bush or shrub, out of whose root a mass of independent branches shoot up in different directions. From this densely branched shrub, most of the branches of which are dead, there rise four high stems with many branches. These are the four lofty trees just mentioned as representing the higher phyla—the Echinoderma, Articulata, Mollusca, and Vertebrata. These four stems are directly connected with one another at the root only, to wit, by the common primary group of the Worm tribe.

The extraordinary difficulties which the systematic arrangement of Worms presents, for this reason merely, are still more increased by the fact that we do not possess any fossil remains of them. Most of the Worms had and still have such soft bodies that they could not leave any characteristic traces in the neptunic strata of the earth. Hence in this case again we are entirely confined to the records of creation furnished by ontogeny and comparative anatomy. In making then the exceedingly difficult attempt to throw a few hypothetical rays of light upon the obscurity of the pedigree of Worms, I must therefore expressly remark that this sketch, like all similar attempts possesses only a provisional value.

The numerous classes distinguished in the tribe of Worms, and which almost every zoologist groups and defines according to his own personal views, are, in the first place, divided into two essentially different groups or branches, which in my Monograph of the Calcareous Sponges I have termed Acœlomi and Cœlomati. For all the lower Worms which are comprised in the class of Flat-worms (Platyhelminthes), (the Gliding-worms, Sucker-worms, Tape-worms), differ very strikingly from other Worms, in the fact that they possess neither blood nor body-cavity (no cœlome); they are, therefore, called Acœlomi. The true cavity, or cœlome, is completely absent in them as in all the Zoophytes; in this important respect the two groups are directly allied. But all other Worms (like the four higher tribes of animals) possess a genuine body-cavity and a vascular system connected with it, which is filled with blood; hence we class them together as Cœlomati.

The main division of Bloodless Worms (Acœlomi) contains, according to our phylogenetic views, besides the still living Flat-worms, the unknown and extinct primary forms of the whole tribe of Worms, which we shall call the Primæval Worms (Archelminthes). The type of these Primæval Worms, the ancient Prothelmis, may be directly derived from the Gastræa (p. [133]). Even at present the Gastrula-form—the faithful historical portrait of the Gastræa—recurs in the ontogenesis of the most different kinds of worms as a transient larva-form. The ciliated Gliding-worms (Turbellaria), the primary group of the present Planary or Flat-worms (Platyhelminthes), are the nearest akin to the Primæval Worms. The parasitical Sucker-worms (Trematoda) arose out of the Gliding-worms, which live freely in water, by adaptation to a parasitical mode of life; and out of them later on—by an increasing parasitism—arose the Tape-worms (Cestoda).

Out of a branch of the Acœlomi arose the second main division of the Worm tribe, the Worms with blood and body-cavity (Cœlomati): of these there are seven different classes.

The Pedigree on p. [151] shows how the obscure phylogeny of the seven classes of Cœlomati may be supposed to stand. We shall, however, mention these classes here quite briefly, as their relationships and derivation are, at present, still very complicated and obscure. More numerous and more accurate investigations of the ontogeny of the different Cœlomati will at some future time throw light upon their phylogenesis.

SYSTEMATIC VIEW
Of the 8 Classes and 22 Orders of the Worm Tribe.
(Compare Gen. Morph. II. Plate V. pp. 75-77.)
Classes of the Worm Tribe.	Orders of the Worm Tribe.		Systematic Name of the Orders of Worms.		Name of a Genus as example.

1. Flat Worms Platyhelminthes	1.	Primæval worms	1.	Archelminthes	Prothelmis
	2.	Gliding-worms	2.	Turbellaria	Planaria
	3.	Sucker-worms	3.	Trematoda	Distoma
	4.	Tape-worms	4.	Cestoda	Tænia

2.Round Worms Nemathelminthes	5.	Arrow-worms	5.	Chætognatha	Sagitta
	6.	Thread-worms	6.	Nematoda	Trichina
	7.	Hook-headed-worms	7.	Acanthocephala	Echinorhynchus

3. Moss Polyps Bryozoa	8.	Horse-shoe-lipped	8.	Lophopoda	Alcyonella
3. Moss Polyps Bryozoa	9.	Circle-lipped	9.	Stelmopoda	Retepora

4. Sea-sacs Tunicata	10.	Sea-squirts	10.	Ascidia	Phallusia
4. Sea-sacs Tunicata	11.	Sea-barrels	11.	Thaliacea	Salpa

5. Proboscideans Rhynchocœla	12.	Tongue-worms	12.	Enteropneusta	Balanoglossus
5. Proboscideans Rhynchocœla	13.	Cord-worms	13.	Nemertina	Borlasia

6. Star-Worms Gephyrea	14.	Star-worms without bristles	14.	Sipunculida	Sipunculus
6. Star-Worms Gephyrea	15.	Star-worms with bristles	15.	Echiurida	Echiurus
7. Wheel Animalcule Rotifera	16.	Wheel-worms	16.	Rotatoria	Hydatina
8. Ring Worms Annelida	17.	Bear-worm	17.	Arctisca	Macrobiotus
	18.	Worms with claws	18.	Onychophora	Peripatus
	19.	Leeches	19.	Hirudinea	Hirudo
	20.	Land-worms	20.	Drilomorpha	Lumbricus
	21.	Mailed worms	21.	Phracthelminthes	Crossopodia
	22.	Bristle-footed worms	22.	Chætopoda	Aphrodite