STAR FISHES. SECOND GENERATION. WORM STOCK. Pl. ix.

The three other classes of Echinoderma evidently arose at a later period out of the class of Sea-stars which have most faithfully retained the original form of the stellate colony of worms. The Sea-lilies, or Crinoida, differ least from them, but having given up the free, slow motion possessed by other Sea-stars, they have become adherent to rocks, etc., and form for themselves a long stalk. Some Encrinites, however (for example, the Comatulæ, Fig. B, on Plates [VIII]. and [IX].), afterwards detach themselves from their stalk. The original worm individuals in the Crinoida are indeed no longer preserved in the same independent condition as in the case of the common star-fish; but they nevertheless always possess articulated arms extending from a common central disc. Hence we may unite the Sea-lilies and Sea-stars into a main-class, or branch, characterized as possessing articulated arms (Colobrachia).

In the other two classes of Echinoderma, the Sea-urchins and Sea-cucumbers, the articulated arms are no longer present as independent parts, but, by the increased centralization of the stock, have completely fused so as to form a common, inflated, central disc, which now looks like a simple box or capsule without arms. The original stock of five individuals has apparently degenerated to the form-value of a simple individual, a single person. Hence we may represent these two classes as a branch characterized as being without arms (Lipobrachia), equivalent to those which possess articulated arms. The first of these two classes, that of Sea-urchins (Echinida) takes its name from the numerous and frequently very large thorns which cover the hard shell, which is itself artistically built up of calcareous plates. (Fig. C, Plates [VIII]. and [IX].) The fundamental form of the shell itself is a pentagonal pyramid. The Sea-urchins probably developed directly out of the group of Sea-stars. The different classes and orders of marine lilies and stars which are given in the following table, illustrate the laws of progress and differentiation in a striking manner. In each succeeding period of the earth’s history we see the individual classes continually increasing in variety and perfection. (Gen. Morph. ii. Plate IV.)

The history of three of these classes of Star-fish is very minutely recorded by numerous and excellently preserved fossils, but on the other hand, we know almost nothing of the historical development of the fourth class, that of the Sea-cucumbers (Holothuriæ). These curious sausage-shaped Star-fish manifest externally a deceptive similarity to worms. (Fig. D, Plates [VIII]. and [IX].) The skeletal structures in their skin are very imperfect, and hence no distinct remains of their elongated, cylindrical, worm-like body could be preserved in a fossil state. However, from the comparative anatomy of the Holothuriæ, we can infer that they have arisen, by the softening of the cutaneous skeleton, from members of the class of Sea-urchins.

From the Star-fish we turn to the fifth and most highly developed tribe of the invertebrate animals, namely, the phylum of Articulata, or those with jointed feet (Arthropoda). As has already been remarked, this tribe corresponds to Linnæus’ class of Insects. It contains four classes: (1) the genuine six-legged Insects, or Flies; (2) the eight-legged Spiders; (3) the Centipedes, with numerous pairs of legs; and (4) the Crabs, or Crustacea, whose legs vary in number. The last class breathe water through gills, and may therefore be contrasted as the main-class of gill-breathing Arthropoda, or Gilled Insects (Carides), with the three first classes. The latter breathe air by means of peculiar wind-pipes, or tracheæ, and may therefore appropriately be united to form the main-class of the trachea-breathing Arthropoda, or Tracheate Insects (Tracheata).

In all animals with articulated feet, as the name indicates, the legs are distinctly articulated, and by this, as well as by the strong differentiation of the separate parts of the body, or metamera, they are sharply distinguished from Ringed worms, with which Bär and Cuvier classed them. They are, however, in every respect so like the Ringed worms that they can scarcely be considered altogether distinct from them. They, like the Ringed worms, possess a very characteristic form of the central nervous system, the so-called ventral marrow, which commences in a gullet-ring encircling the mouth. From other facts also, it is evident that the Arthropoda developed at a late period out of articulated worms. Probably either the Wheel Animalcules or the Ringed worms are their nearest blood relations in the Worm tribe. (Gen. Morph. ii. Plate V. pp. 85-102.)

Now, although the derivation of the Arthropoda from ringed Worms may be considered as certain, still it cannot with equal assurance be maintained that the whole tribe of the former has arisen out of one branch of the latter. For several reasons seem to support the supposition that the Gilled Arthropods have developed out of a branch of articulated worms, different from that which gave rise to the Tracheate Arthropods. But on the whole it remains more probable that both main-classes have arisen out of one and the same group of Worms. In this case the Tracheate Insects—Spiders, Flies, and Centipedes—must have branched off at a later period from the gill-breathing Insects, or Crustacea.

The pedigree of the Arthropoda can on the whole be clearly made out from the palæontology, comparative anatomy, and ontogeny of its four classes, although here, as everywhere else, many details remain very obscure. Not until the history of the individual development of all the different groups has become more accurately known than it is at present, can this obscurity be removed. The history of the class of Gilled Insects, or Crabs (Carides), is at present that best known to us; they are also called encrusted animals (Crustacea), on account of the hard crust or covering of their body. The ontogeny of these animals is extremely interesting and, like that of Vertebrate animals, distinctly reveals the essential outlines of the history of their tribe, that is, their phylogeny. Fritz Müller, in his work, “Für Darwin,”[(16)] which has already been referred to, has explained this remarkable series of facts in a very able manner.

Nauplius. Youth-form of six Crab-fish.Pl. X.


Adult form of the same six Crab-fish.		Pl. XI.

The common primary form of all Crabs, which in most cases is even now the first to develop out of the egg, is originally one and the same, the so-called Nauplius. This remarkable primæval crab represents a very simple form of articulated animal, the body of which in general has the form of a roundish, oval, or pear-shaped disc, and has on its ventral side only three pairs of legs. The first of these is uncloven, the two subsequent pairs are forked. In front, above the mouth, lies a simple, single eye. Although the different orders of the Crustacean class differ very widely from one another in the structure of their body and its appendages, yet the early Nauplius form always remains essentially the same. In order to be convinced of this, let the reader look attentively at Plates [X]. and [XI]., a more detailed explanation of which is given in the Appendix. On Plate [XI]. we see the fully developed representatives of six different orders of Crabs, a Leaf-footed Crab (Limnetis, Fig. A c); a Stalked Crab (Lepas, Fig. D c); a Root Crab, (Sacculina, Fig. E c); a Boatman Crab (Cyclops, Fig. B c); a Fish Louse (Lernæocera, Fig. C c); and, lastly, a highly developed Shrimp (Peneus, Fig. F c). These six crabs vary very much, as we see, in the entire form of body, in the number and formation of the legs, etc. When, however, we look at the earliest stages, or “nauplius,” of these six different classes, after they have crept out of the egg—those marked with corresponding letters on Plate [X]. (Fig. A n-F n)—we shall be surprised to find how much they agree. The different forms of Nauplius of these six orders differ no more from one another than would six different “good species” of one genus. Consequently, we may with assurance infer a common derivation of all those orders from a common Primæval Crab, which was essentially like the Nauplius of the present day.

SYSTEMATIC SURVEY
Of the 7 Legions and 20 Orders of Crabs, or Crustacea.
Legions of the
Crustaceæ.		Orders of the
Crustaceæ.		Systematic name
of the Orders.		Name of a
Genus as an
example.
I. Entomostraca, Lower Crustacea, or Segmented Crabs (not passing through theactual Zoëa form in youth).
I. Branchiopoda
Gill-footed Crabs

1.Primæval Crabs1.ArchicaridaNauplius
2.Leaf-foot Crabs2.PhyllopodaLimnetis
3.Trilobites3.TrilobitaParadoxides
4.Water Fleas4.CladoceraDaphnia
5.Bivalve Crabs5.OstracodaCypris
II. Pectostraca
Fixed Crabs

6.Barnacle Crabs6.CirripediaLepas
7.Root Crabs7.RhizocephalaSacculina
III. Copepoda
Oar-footed Crabs

8.Boatmen Crabs8.EucopepodaCyclops
9.Fish Lice9.SiphonostomaLernæocera
IV. Pantopoda
No-body Crabs

10.No-body Crabs10.PycnogonidaNymphon
V. Pœcilopoda
Oar-footed Crabs

11.Spear-tails11.XiphosuraLimulus
12.Giant Crabs12.GigantostracaEurypterus
II. Malacostraca, Higher Crustacea, or Mailed Crabs (passing through the Zoëa formin youth).
V. Pœcilopoda
Oar-footed Crabs

13.Zoëa Crabs13.ZoëpodaZoëa
14.Split-legged Crabs14.SchizopodaMysis
15.Mouth-footed Crabs15.StomatopodaSquilla
16.Ten-footed Crabs16.DecapodaPeneus
VI. Edriophthalma
Mailed Crabs with sessile
eyes

17.Cuma Crabs17.CumaceaCuma
18.Flea Crabs18.AmphipodaGammarus
19.Wizard Crabs19.LæmodipodaCaprella
20.Crabs20.IsopodaOniscus