Archer Shee, The Life of Sir M. A. Shee, P.R.A. (London, 1860); C. R. Leslie, R.A., and Tom Taylor, Life and Times of Sir Joshua Reynolds, P.R.A. (London, 1865); J. E. Hodgson, R.A. (the late), and Fred. A. Eaton, Sec. R.A., “The Royal Academy in the Last Century,” Art Journal, 1889–1891. But the chief sources of information on the subject are the minute-books of the council and of the general assembly, and the annual reports, which, however, only date from 1859. (F. A. E.)
ACADIAN, in geology, the name given by Sir J. W. Dawson in 1867 to a series of black, red and green shales and slates, with dark grey limestones, which are well developed at St John, New Brunswick; Avalon in E. Newfoundland, and Braintree in E. Massachusetts. These rocks are of Middle Cambrian age and possess a Paradoxides fauna. They have been correlated with limestone beds in Tennessee, Alabama, Central Nevada and British Columbia (St Stephen).
See Cambrian System; also C. D. Walcott, Bull. U.S. Geol. Survey, No. 81, 1891; and Sir J. W. Dawson, Acadian Geology, 1st ed. 1855, 3rd ed. 1878.
ACADIE, or Acadia, a name given by the French in 1603 to that part of the mainland of North America lying between the latitudes 40° and 46°. In the treaty of Utrecht (1713) the words used in transferring the French possessions to Britain were “Nova Scotia or Acadia.” See Nova Scotia for the limits included at that date under the term.
ACANTHOCEPHALA, a compact group of cylindrical, parasitic worms, with no near allies in the animal kingdom. Its members are quite devoid of any mouth or alimentary canal, but have a well-developed body cavity into which the eggs are dehisced and which communicates with the exterior by means of an oviduct. The size of the animals varies greatly, from forms a few millimetres in length to Gigantorhynchus gigas, which measures from 10 to 65 cms. The adults live in great numbers in the alimentary canal of some vertebrate, usually fish, the larvae are as a rule encysted in the body cavity of some invertebrate, most often an insect or crustacean, more rarely a small fish. The body is divisible into a proboscis and a trunk with sometimes an intervening neck region. The proboscis bears rings of recurved hooks arranged in horizontal rows, and it is by means of these hooks that the animal attaches itself to the tissues of its host. The hooks may be of two or three shapes. Like the body, the proboscis is hollow, and its cavity is separated from the body cavity by a septum or proboscis sheath. Traversing the cavity of the proboscis are muscle-strands inserted into the tip of the proboscis at one end and into the septum at the other. Their contraction causes the proboscis to be invaginated into its cavity (fig. 2). But the whole proboscis apparatus can also be, at least partially, withdrawn into the body cavity, and this is effected by two retractor muscles which run from the posterior aspect of the septum to the body wall (fig. 3).
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| From Cambridge Natural History, vol. ii., “Worms, &c.,” by permission of Macmillan & Co., Ltd. |
Fig. 1. |
| A, Five specimens of Echinorhynchus acus, Rud., attached to a piece of intestinal wall, |
| B, The proboscis of one still more highly magnified. |
The skin is peculiar. Externally is a thin cuticle; this covers the epidermis, which consists of a syncytium with no cell limits. The syncytium is traversed by a series of branching tubules containing fluid and is controlled by a few wandering, amoeboid nuclei (fig. 2). Inside the syncytium is a not very regular layer of circular muscle fibres, and within this again some rather scattered longitudinal fibres; there is no endothelium. In their minute structure the muscular fibres resemble those of Nematodes. Except for the absence of the longitudinal fibres the skin of the proboscis resembles that of the body, but the fluid-containing tubules of the latter are shut off from those of the body. The canals of the proboscis open ultimately into a circular vessel which runs round its base. From the circular canal two sac-like diverticula called the “lemnisci” depend into the cavity of the body (fig. 2). Each consists of a prolongation of the syncytial material of the proboscis skin, penetrated by canals and sheathed with a scanty muscular coat. They seem to act as reservoirs into which the fluid of the tense, extended proboscis can withdraw when it is retracted, and from which the fluid can be driven out when it is wished to expand the proboscis.
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From Cambridge Natural History, vol. ii., “Worms, &c.,” by permission of Macmillan & Co., Ltd. | |
Fig. 2.—A longitudinal section through the anterior end of Echinorhynchus haeruca, Rud. (from Hamann). | |
| a, | The proboscis not fully expanded. |
| b, | Proboscis-sheath. |
| c, | Retractor muscles of the proboscis. |
| d, | Cerebral ganglion. |
| e, | Retinaculum enclosing a nerve. |
| f, | One of the retractors of the sheath. |
| g, | A lemniscus. |
| h, | One of the spaces in the sub-cuticular tissue. |
| i, | Longitudinal muscular layer. |
| j, | Circular muscular layer. |
| k, | Line of division between the sub-cuticular tissue of the trunk and that of the proboscis with the lemnisci. |
There are no alimentary canal or specialized organs for circulation or for respiration. Food is imbibed through the skin from the digestive juices of the host in which the Acanthocephala live.
J. Kaiser has described as kidneys two organs something like minute shrubs situated dorsally to the generative ducts into which they open. At the end of each twig is a membrane pierced by pores, and a number of cilia depend into the lumen of the tube; these cilia maintain a constant motion.
The central ganglion of the nervous system lies in the proboscis-sheath or -septum. It supplies the proboscis with nerves and gives off behind two stout trunks which supply the body (fig. 2). Each of these trunks is surrounded by muscles, and the complex retains the old name of “retinaculum.” In the male at
