All the Errant Annelides are marine, occurring in all seas from the Arctic Ocean to the equator, and extending to great depths. A few forms (e.g., Tomopteris) are pelagic. Others live in sand and mud; whilst others hide under stones, or in fissures in rock-pools; and others, again, bore holes in calcareous rocks. A few live as "commensals" on other animals.

Fig. 119.-Errant Annelides. A, Hairy-bait (Nephthys); B, Sea-mouse (Aphrodite); C, Lob-worm (Arenicola). (After Gosse.)

DISTRIBUTION OF ANNELIDA IN TIME.-Of the Annelida the only orders which are known to have left any traces of their existence in past time are the Tubicola and the Errantia; of which the former are known by their investing tubes, whilst the latter are only recognised by the tracks which they left upon ancient sea-bottoms, or by their burrows in sand or mud. These tracks and burrows of Annelides are found commonly in rocks of almost all ages from the Cambrian period upwards. Those tracks which have been caused simply by the passage of the worm over the surface of the mud are termed by Mr Salter Helminthites, whilst the burrows are called Scolites (or Scolithus).

Tubicolar Annelides are known to occur from the Silurian rocks upwards. The well-known Silurian fossil Tentaculites, has been often referred to the Tubicola, but is almost certainly Pteropodous. Cornulites, Serpulites, Ortonia, Trachyderma, Spirorbis, and Conchicolites are, however, genuine Silurian Tubicola. The Microconchus carbonarius is a little spiral Tubi

colar Annelide, nearly allied to the Spirorbis (fig. 116, b) of our seas, which is not uncommonly found in strata belonging to the Carboniferous period; and the genus Spirorbis itself is represented even in the Silurian period.

CLASS III. CHATOGNATHA (Huxley).-Elongated cylindrical animals having the hinder extremity of the body furnished with an integumentary fin. Anterior end of the body provided with setæ and corneous jaws. No foot-tubercles. Sexes united in the same individual.

This class includes only the singular pelagic animals belonging to the genus Sagitta, the precise systematic position of which is somewhat doubtful. They appear, however, to form a connecting link between the Annelides on the one hand, and the free Nematoids on the other hand.

The Sagitta (fig. 120) have elongated transparent bodies,

Fig. 120. Morphology of Chatognatha. A, Sagitta tricuspidata of the natural size; o One of the ovaries; sp Orifice of one of male organs of reproduction. B, Head of the same, viewed from beneath and greatly enlarged, showing the horny, setiform jaws. (After Saville Kent.)

rarely over an inch in length, having the hinder end of the body expanded into a striated caudal fin, similar fins often existing on the sides of the body as well. The head carries a series of setæ placed in front of the mouth, and the oral aperture is furnished with unciform corneous bristles or "falces," which act as jaws. The alimentary canal is straight, and terminates in an anus placed at the base of the tail below.

"A single oval ganglion lies in the abdomen, and sends, forwards and backwards, two pairs of lateral cords. The lateral cords unite in front of and above the mouth into a hexagonal ganglion. This gives off two branches which dilate at their extremities into the spheroidal ganglia, on which the

darkly pigmented imperfect eyes rest. The ovaries, saccular organs, lie on each side of the intestine and open on either side of the vent; receptacula seminis are present. Behind the anus, the cavity of the tapering caudal part of the body is partitioned into two compartments; on the lateral parietes of these, cellular masses are developed which become detached, and, floating freely in the compartment, develop into spermatozoa. These escape by spout-like lateral ducts, the dilated bases of which perform the part of vesiculæ seminales. The embryos are not ciliated, and undergo no metamorphosis " (Huxley).

The species of Sagitta are found, living in the open sea, in the Mediterranean, and in the Atlantic and Pacific Oceans.

LITERATURE.
GEPHYREA.

1. "History of British Starfishes and other Animals of the Class Echino. dermata." Edward Forbes. 1841.

2. "Mémoire sur l'Echiure." Quatrefages. 'Annales Sci. Nat.' 1847. 3. "Ueber Thalassema." Krohn. Arch. für Anat. und Physiologie.' 1842.

4. "Observationes anatomicæ de vermibus quibusdam maritimis." Müller. 1852. 5. "Larven von Phascolosoma." Selenka. 'Zeitschrift Wiss. Zool.' 1875.

6. "

ANNELIDA.

Monographie de la Famille des Hirudinés." Moquin-Tandon. 1846.

7. "De lumbrici terrestris historia naturali, nec non anatomia." Morren. 1829.

8. "Observations on the Organisation of Oligochatous Annelides." Ray Lankester. 'Annals of Nat. Hist.' 1871.

9.

"Structure of Tubifex." W. C. M'Intosh. 'Trans. Roy. Soc. Edin.' 1870-71.

10. Article "Annelides," in "Todd's Cyclopædia of Anatomy and Physiology.' Milne-Edwards. 1835.

11. Article "Annelida," in Encyclopædia Britannica,' 9th ed. vol. ii.

W. C. M'Intosh. 1875.

Annales Sci. Nat.' 1832-33.

Système des Annélides.

Savigny. 1820.

13.

"Classification des Annélides," &c.

Audouin and Milne-Edwards.

14.

15

"Études sur les types inférieures de l'embranchement des Annélés." Quatrefages. 'Annales Sci. Nat.' 1848-54.

"Histoire Naturelle des Annélés." Quatrefages.

16. “Zur Anatomie und Physiologie der Kiemenwürmer." Grube. 1838. 17. "Die Familien der Anneliden." Grube.

schichte.' 1850.

'Archiv für Naturge

Claparède. 1861.

18. Recherches anatomiques sur les Annélides." 19. "Recherches anatomiques sur les Oligochétes." Claparède. 1862.

R

20. "Structure des Annélides sédentaires." Claparède. 1873. 21. "Die Borstenwürmer." Ehlers. 1864 and 1868.

22. "Nordiske Haf's Annulater." Malmgren. 1866.

23.

24.

"Annulata polychaeta Spitzbergiæ," &c. Malmgren. 1867.

'Catalogue of the British Non-Parasitical Worms in the Collection of the British Museum." Johnston. 1865-67.

25. "Les Annélides Chétopodes du Golfe de Naples." Claparède. 1868. Supplement, 1870.

26. "On the Young Stages of a few Annelides." Alexander Agassiz. Ann. Lyceum Nat. Hist.' New York. 1866.

27. "Christiania-Fjords Fauna. Anneliden." O. Sars. 1873.

28. "Life-Histories of Animals." Packard. "Grönland's Annulata dorsibranchiata." Skriften.' 1843.

29.

30.

31.

CHATOGNATHA.

1876.

Oersted. 'Danske Selsk.

Species of Sagitta." Busk. 'Quart. Journ. Microscop. Science.' 1856.

"Ueber die Entwickelung der Sagitta." Gegenbaur. 1857.

32. "On a New Species of Sagitta from the South Pacific. Saville Kent. 'Annals Nat. Hist.' 1870.

CHAPTER XXX.

ARTHROPODA.

DIVISION II. ARTHROPODA, OR ARTICULATA.-The remaining members of the sub-kingdom Annulosa are distinguished by the possession of jointed appendages, articulated to the body; and they form the second primary division-often called by the name Articulata. As this name, however, has been employed in a wider sense than is understood by it here, it is perhaps best to adopt the more modern term Arthropoda.

The members of this division, comprising the Crustacea (Lobsters, Crabs, &c.), the Arachnida (Spiders and Scorpions), the Myriapoda (Centipedes), and the Insecta, are distinguished

as follows:

The body (fig. 110) is composed of a series of segments, arranged along a longitudinal axis; each segment, or "somite," occasionally, and some always, being provided with articulated appendages. Both the segmented body and the articulated limbs are more or less completely protected by a chitinous exoskeleton, formed by a hardening of the cuticle. The appendages are hollow, and the muscles are prolonged into their interior. The nervous system in all, at any rate in the embryonic condition, consists of a double chain of ganglia, placed

along the ventral surface of the body, united by longitudinal commissures, and traversed anteriorly by the oesophagus. The hæmal system, when differentiated, is placed dorsally, and consists of a contractile cavity, or heart, provided with valvular apertures, and communicating with a perivisceral cavity, containing corpusculated blood. Respiration is effected by the general surface of the body, by gills, by pulmonary sacs, or by tubular involutions of the integument, termed "trachea." In no member of the division are vibratile cilia known to be developed. According to Professor Huxley, an additional constant character of the Arthropoda is to be found in the structure of the head, which is typically composed of six segments, and never contains less than four.

The Arthropoda are divided into four great classes-viz., the Crustacea, the Arachnida, the Myriapoda, and the Insecta; which are roughly distinguished as follows:

1. CRUSTACEA.- Respiration by means of gills, or by the general surface of the body. Two pairs of antenna. Locomotive appendages, more than eight in number, borne by the segments of the thorax, and usually of the abdomen also.

2. ARACHNIDA.-Respiration by pulmonary vesicles, by trachea, or by the general surface of the body. Head and thorax united into a cephalothorax. Antenna (as such) absent. Legs eight. Abdomen without articulated appendages.

3. MYRIAPODA.—Respiration by trachea. Head distinct; remainder of the body composed of nearly similar somites. One pair of antenna. Legs numerous.

4. INSECTA.-Respiration by trachea. abdomen distinct. One pair of antennæ. borne on the thorax. Abdomen destitute two pairs of wings on the thorax.

Head, thorax, and Three pairs of legs of limbs. Generally

CHAPTER XXXI.

CRUSTACEA.

CLASS I. CRUSTACEA.-The members of this class are commonly known as Crabs, Lobsters, Shrimps, King-crabs, Barnacles, Acorn-shells, &c. They are nearly allied to the succeeding order of the Arachnida (Spiders and Scorpions); but may usually be distinguished by the possession of articulated