a large number of genera), it is hardly probable that mere branchial feet would serve for their locomotion.

If, however, each free and movable thoracic segment was furnished with a pair of appendages, as among the modern Isopods, and as is also indicated in the larva of the earliest stages of development within the egg of the modern Limulus (See Plate XCI., figs. 17-19), then another point is gained in our investigation, and we see that the earliest embryonal stages are those which naturally foreshadow the earliest and simplest adult forms. In other words, all the immense variety of forms in a group are but the expression of the sum of the stages passed through by the highest individual in arriving at perfection

Another relation which the Merostomata and Trilobita exhibit, and upon which much stress has been laid by Dr. Dohrn and Prof. Hæckel, is that between these palæozoic types and the Arachnida; particularly between the Eurypterida and the Scorpionidæ.

And it is a most significant fact that the earliest Arachnides occur as far back as the Coal-measures, where the last of the Eurypterida and the Trilobita are also met with. Anyone who has examined a scorpion, or is acquainted with its form and structure from books and drawings, cannot fail to be struck by the remarkable resemblance between it and the Eurypterida, even to the arrangement of the appendages, the position of the eyes, &c., &c. Indeed, we may very fairly infer that from this division of the Crustacea the Scorpionidæ of to-day were derived. Nor is there any insuperable difficulty in accepting this view on sound physiological grounds. The possibility of an animal passing through larval conditions, casting aside, at even a single moult, its branchiæ, and assuming aërial respiration, quitting the water and inhabiting the land, changing its element, its diet, its mode of progression, and its entire life, is no chimerical speculation. Such cases are familiar to the entomologist, the carcinologist,† and even to the herpetologist.‡

*

But the acceptance of this proposition does not, as has been assumed by these writers, necessitate the removal of the Eurypterida from the Crustacea; on the contrary, as Fritz Müller well observes, "If all the classes of the Arthropoda (Crustacea, Insecta, Myrapoda, and Arachnida) are indeed all branches of a common stem (and of this there can scarcely be a doubt), it is evident that the water-inhabiting and waterbreathing Crustacea must be regarded as the original stem

The larval and adult Libellula, Ephemera, &c.

† Gecarcinus ruricola, and other land-crabs.

The Batrachia.

VOL. XI.-NO. XLV.

D D

from which the other (terrestrial) classes, with their tracheal respiration, have branched off.”

Viewed as a whole, the Crustacea probably present to us the best zoological illustration of a class constructed on a common type retaining its general characteristics, but capable of endless modifications of its parts, so as to suit the extreme requirements of every separate species.

And it is doubtless in great degree due to this plasticity of structure, enabling the species to occupy such diverse positions, and to subsist upon such varied aliment, that the class owes its preservation through the lapse of ages represented by the long series of geological formations, from the Cambrian strata to the present day.

EXPLANATION OF PLATE XC.

FIG. 1. Pterygotus anglicus, Agassiz, ventral aspect restored. From the Devonian of Forfarshire (about one-sixteenth natural size).

2. Slimonia acuminata, H. Woodw., dorsal aspect restored. From the Upper Silurian, Lesmahagow, Lanarkshire (about oneninth natural size).

3. Stylonurus Logani, H. Woodw., dorsal aspect restored. From the Upper Silurian, Lesmahagow, Lanarkshire (about one-half natural size).

4. Eurypterus Scouleri, Hibbert, dorsal aspect restored. From the Carboniferous limestone of Kirkton quarry and Bathgate, West Lothian (about one-eighth natural size).

5. Hemiaspis limuloides, H. Woodw., dorsal aspect. From the Lower Ludlow Leintwardine, Shropshire (about two-thirds natural size).

6. Prestwichia rotundata, H. Woodw., Coal-measures, Ironstone, Coalbrook Dale (about two-thirds natural size).

7. Bellinurus Königianus, H. Woodw. sp. nov. Coal-measures, Dudley Coal-field, in clay ironstone (natural size).

8. Prestwichia Birtwelli, H. Wood. sp. nov. Coal-measures, Cornfield Pits, near Padiham, Lancashire (natural size).

9. Neolimulus falcatus, H. Woodw. Upper Silurian Lesmahagow, Lanarkshire (twice natural size).

10. Dithyrocaris Scouleri, McCoy, Carboniferous, Ireland (one-fifth natural size).

11. Larva or Zöea of the common "Shore-crab," Carcinus manas, Penn. 2nd stage, copied from Mr. C. Spence Bate's paper on the "Development of Decapod Crustacea," Phil. Trans. 1858, p. 589, Pl. XL.

* "Facts and Arguments for Darwin," by Fritz Müller (p. 120). Translated from the German by W. S. Dallas, F.L.S., &c.

[Figs. 1-9, copied and reduced from the plates illustrating Mr. Henry Woodward's Monograph on the Merostomata, published in the volumes of the Palæontographical Society, 1867–72.]

EXPLANATION OF PLATE XCI.

[This plate is reproduced here by permission of the Council of the Geological Society of London from a paper by Mr. H. Woodward, “On the Relationship of the Xiphosura to the Eurypterida, and to the Trilobita and Arachnida." See "Quart. Journ. Geol. Soc.," 1872. Vol. XXVIII. p. 46.]

FIGS. 1-8. Trinucleus ornatus, Sternb. sp. (copied from Barrande's "Système Silurien du centre de la Bohême," Prague, 1852, 4to, plate 30). Specimens arranged in series according to their supposed age. (All the stages figured by Barrande are not given here.)

FIG. 1. Young individual, destitute of thoracic segments, composed of head-shield and pygidium only.

2. Another of the same stage, in which the genal or cheek-spines are

developed.

3. Individual with one thoracic segment developed, but without the

4.

5.

genal spines.

Another of the same stage, with the genal spines.

Individual with two thoracic segments, and in which the genal spines are present.

6. Individual with three thoracic segments, and possessing the genal

7.

8.

spines.

Individual with five thoracic segments, but without genal spines. Adult Trinucleus, with six thoracic segments and fully-developed genal spines.

FIGS. 9-15. Sao hirsuta, Barrande (copied from plate 7 of Barrande's work above cited). Barrande figures twenty stages of this Trilobite, of which we have only reproduced seven.

FIG. 9. First stage. A young individual in which the limit of the headshield is not indicated as separating it from the pygidium.

10. Second stage. Young individual with the head-shield separated, and having inclinations of three soldered segments to the pygidium.

11. Third stage, in which the genal angles of the head and the spiny border of the pygidium are well seen, and four or five soldered segments indicated.

12. Fourth stage, in which two free thoracic segments are developed behind the head, and two or three soldered segments represent the pygidium.

13. Fifth stage, in which the thorax is longer than the head, and is composed of three movable segments and three soldered segments in the pygidium.

14. Sixth stage, in which four free segments succeed the head, and three or four soldered segments form the pygidium.

FIG. 15. Tenth stage, in which eight free segments succeed the head, and three soldered segments form the pygidium. [In the twentieth stage figured by Barrande the adult has seventeen free thoracico-abdominal segments and two soldered ones (the pygidium).]

16.

Egg of Limulus polyphemus: a, the chorion; b, the exochorian (after Dohrn *).

17. Third stage in the embryo of Limulus: a, chorion; b, exochorion (after Packard †).

18.

Fourth stage (?) in the embryo of Limulus (after Dr Packard's † figure).

19. Fourth stage (?) in the embryo of Limulus: 1, antennules; 2, antenna; 3-6, maxillipeds; 7 and 8, thoracic plates afterwards bearing the branchiæ; m, the mouth; x, the ovarian apertures (?); a, the abdomen (after Dohrn *).

20. Fifth stage (?) of embryo of Limulus (after Dohrn*). At this stage the exochorion is split, and the chorion is expanded by the admission of water by endosmose, in which the embryo is seen to revolve.

21. Ninth stage () of embryo, just before hatching' (after Packard †): dorsal aspect.

22.

23.

24.

The same side view of embryo.

Larva of Limulus recently hatched (after Packard †).

Larva of Limulus on hatching (the "Trilobitenstadium” of
Dohrn *).

"Zur Embryologie und Morphologie des Limulus polyphemus." Von Dr. Anton Dohrn. (Jenaische Zeitschrift, Band vi. Heft 4, p. 580, Tafeln xiv. xv.) Received September 30, 1871.

"On the Embryology of Limulus polyphemus." By A. S. Packard, jun., M.D. Read before the American Association for the Advancement of Science, August 1870. ("American Naturalist," vol. iv. No. 8, 1870, October, p. 498.)