Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/696

662 CRUSTACEA Palæozoic rocks. Several other Amphipod-like forms occur in the lithographic stone of Bavaria. The world-wide distribution of the Amphipoda accords well with their range in time, which was as great or even greater than the Isopoda.

Sub-class 2. Gnathopoda (or Entomostraca).

III. Merostomata: (6.) Xiphosura. – The king-crab (Limulus) is a remarkable type of crustacean closely related to the extinct Eurypterida. Found living in the seas of China and Japan and on the north-east coast of North America, it exemplifies a peculiar and most ancient order, the affinities of which are not at first readily recognized because its nearest allies have passed away. The head-shield is enormously expanded so as to shelter all the anterior appendages beneath it; and the succeeding segments are so soldered together as to appear like one piece, although all the hind-segments are free and movable in the larva. The eyes are fixed on the head-shield; the antennules are chelate, and placed in front of the mouth. The antennae, mandibles, maxillae, maxillipeds, are all con verted into walking legs, forming also chelate appendages and, at their bases, jaws; thus serving admirably to illus trate the most prominent characteristic of the sub-class Gnathopoda, "mouth-footed."

Fig. 80. – 1, Limulus polyphemus, adult (dorsal aspect). 2, Limulus polyphemus, young (dorsal aspect). 3, Prestwichia rotundata, Coal M., Shropshire. 4, Prestwichia Birtwelli, Coal M., Lancashire. 5, Neolimulus falcatus, U. Silurian, Lanark. 6, Hemiaspis limuloides, L. Ludlow, Leintwardine, Shropshire. 7, Pseudoniscus aculeatus, U. Silurian, Russia.

The thoracic feet are flattened out into broad bilobed plates which cover the branchiæ and the egg-pouches. The abdomen is rudimentary, being partly represented by the posterior portion of the hinder shield, and partly by the long ensiform tail-spine (?) (See Owen's Memoir, Trans. Linn. Soc., vol. xxxviii., 1873.) But in the larvæ, as has been already shown, these post-cephalic somites are free and unanchylosed, and the tail-spine is undeveloped, thus connecting the modern king-crab with its far-off ancestors in the Coal and Silurian periods. The oldest species known is the Neolimulus falcatus, H. Woodw. (5 in fig. 80), from the Upper Silurian of Lanarkshire, in which the segments are apparently all free and unanchylosed.

In the Coal-measures no fewer than three genera and eight species of small Limuloid Crustaceans have been met with, viz. Bellinurus (four species), Prestwichia (three species), and Euproops (one species), – the last named an American form. Many of these closely resemble young larval Limuli. The Oolitic Limuli found in the lithographic stone of Solenhofen agree closely with existing species, one form even equalling in size the living Limulus polyphemus from the American coast (1 in fig. 80; see also fig. 12).

III. Merostomata: (7.) Eurypterida. – In this order we become acquainted with the second extinct type of the Crustacean class, and by far the most interesting, be cause all the append ages as well as the body- rings have been preserv ed to us, whereas in the Trilobita the former are remarkable by their almost entire absence.

Unlike Limulus, in which all the segments in the adult are soldered together into a fore and hind body and telson, in the Eurypterida the body is long and well adapted for swimming, the segments being quite distinct and well developed; the feet are also fitted for natation (see 5 in fig. 73, and fig. 81).

Fig. 81. – Underside of Pterygolus Anglicus, Ag. (restored), c, Cephalon; m, Metastoma or post-oral plate. 1, The compound eyes; 2, Chelate antennæ; 3, The mandibles; 4, First maxillæ; 5, Second maxillæ; 6, Maxillipeds; 7, The operculum or thoracic plate, which fits closely against the ventral surfaces of the two anterior thoracic somites, 8 and 9. 8-14, Thoracic somites; 15-19, Abdominal somites; 20, Telson.

We again observe the reiteration of the same well-marked characteristics in the legion Merostomata – already noticed in the Edriophthalmia and Podophthalmia; namely, the division into Brachyuran and Macrouran forms which exemplify the crawling and swimming types, by the soldering together of the body-segments in the one and the retention of free movement in the somites in the other. The characters of these two orders of the Merostomata are summarized in the subjoined table.^[1]

Limulus (Fossil and living). 1. Eyes sessile, compound. 2. Ocelli distinctly seen. 3. All the limbs serving as mouth-organs. 4. Metastoma rudimentary. 5. All the thoracic segments bearing branchiæ or reproductive organs. 6. Other segments destitute of any appendages. 7. Thoracic segments anchylosed. 8. Abdominal segments anchylosed and rudimentary. Pterygotus (Fossil, extinct). 1. Eyes sessile, compound. 2. Ocelli distinctly seen. 3. All the limbs serving as mouth-organs. 4. Metastoma large. 5. Anterior thoracic segments bearing branchiæ or reproductive organs. 6. Other segments destitute of any appendages. 7. Thoracic segments unanchylosed. 8. Abdominal segments free and well developed.