AMPHINEURA, group of marine invertebrate animals con stituting a class of the Mollusca (q.v.), the majority with a shell of eight transverse plates. A much smaller number are de void of a shell and have the general appearance of worms. Up to the present upwards of 120 genera of these animals have been described. They are all sedentary, slow-moving creatures usually of insignificant size, and are generally found in shallow water. Until recent years it was customary to regard the class as divisible into two orders, the shell-bearing Chitons or Polyplacophora and the worm-like Solenogastres or Aplacophora. The Chitons are divided into three suborders according to the structure of the shell, and the Solenogastres into two suborders, the Neomenio morpha- and Chaetodermomorpha. Some zoologists, however, re gard the Solenogastres as more closely allied to certain groups of primitive worms (Platyhelmia, Archiannelida) than to the Mol lusca. J. Thiele (1925) has accordingly removed them from the Mollusca, in which they were placed by Spengel and Ray Lankester. The name Amphineura, which was used to designate the Chitons and Solenogastres collectively, is therefore aban doned by Thiele and the older name Loricata has been revived for the Chitons, which are still treated by Thiele as a class of the Mollusca. In this article the Solenogastres are included in the Mollusca and the unity of the Amphineura is upheld ; but the question of the relationships of the Solenogastres cannot be re garded as finally settled. (See below.) The Chitons are elongate, rather flattened animals, rarely ex ceeding a few inches in length. Cryptochiton stelleri attains a length of 2ocm., and the smallest forms barely exceed half an inch when fully grown. The overlapping plates of the shell give them the appearance of woodlice, a comparison perhaps more accurate than that involved in the German name Kaferschnecke, or "beetle-snails." The outer covering of the animal constitutes the "mantle" on the upper surface and sides. On the lower sur face it forms the solid and muscular foot. (See MoLLUSCA. ) The cells of the mantle secrete the eight calcareous plates of the shell. In most genera of Chitons the separate valves are articu lated together, though with enough f re dom to enable the animal to roll up into a ball like a wood-louse. The plates are pierced by a number of branching canals through which nerves pass to end on the surface of the shell in peculiar sense organs known as "shell-eyes." In some forms the shell becomes covered by the mantle. This is well seen in the large Cryptochiton stelleri of the north-east Pacific. The mantle also secretes horny or calcified spicules. These are found at the edge of the mantle (girdle), where it projects beyond the shell. In Tonicia and other genera the spicules are so large and numerous that they give the animal a rough and shaggy appearance. The cavity of the mouth con tains the radula (see MOLLUSCA) or rasping tongue. The ali mentary canal extends in a straight line from the mouth to the anus, which is situated at the posterior end of the body. It re ceives the ducts of the mucous and salivary glands and of the liver. The heart occupies a position in the middle line at the posterior end. A single aorta leads the blood from the heart, and it is eventually collected for oxygenation into two large cavities (sinuses). From these it passes to the gills, which project from the adjacent body-wall into the cavity formed by the edge of the mantle, where it overhangs the sides. The gills vary in number from four to 80. There are two kidneys, one on each side of the body. Their external apertures are at the posterior end of the body, and at their inner end they open into the pericardium (the cavity surrounding the heart). The nervous system consists of four main nerve trunks joined by transverse commissures and, unlike the main nerve cords of Gastropoda and other molluscs, is largely without ganglia. The sexes are separate in the Chitons. The ovary and testis have no communication with the pericardium as they have in the Solenogastres. The ova and spermatozoa are passed to the exterior by a pair of gonaducts.
Regarded as molluscs, the Solenogastres are mainly charac terized by the loss of the shell, the substantial reduction or loss of the foot, and the fact that there are no separate generative ducts. The gonad (ovary or testis) opens into the pericardium and the ova or spermatozoa are discharged by the kidneys. The gills are concentrated in a posterior cavity (cloaca). In Neo menia and its allies there is, on the under surface of the body, a groove 'which is interpreted as the line of junction between the two edges of the mantle which have grown round the sides. In the Cliaetodermatidae this groove is absent, and the animal is entirely cylindrical.
The embryological development of the Solenogastres is still im perfectly known; but, as far as it has been studied, it is held to endorse the conclusion that these animals are more nearly allied to certain groups of worms than to the Mollusca.
As a class the Amphineura have a world-wide distribution; but, while some genera (e.g., Lepidopleurus) are found in nearly all seas, others have a more local ized occurrence. The Chitons are found in a fossil state in the Or dovician. Their evolutionary his tory has been uneventful, no very marked departure from the main type of shell-structure being recorded. T h e Solenogastres have not so far been recognized as fossils. Both the Chitons and the Solenogastres are sluggish animals, creeping slowly about to no great distance. The Chitons are found on or under rocks and stones. The chemical constitution of the rock is of small impor tance in determining the occurrence of these animals. They prefer a smooth surface and are usually found on rocks that weather to such a condition. Ashby found the greatest numbers on pure granite, "counting over 7o specimens of Ischnoradsia on a single rock no larger than a dinner plate." Neomenia and its allies live on corals and hydroids, while Chaetoderma prefers muddy and oozy bottoms. Some members of the class are found at very great depths, e.g., Leptochiton benthus in 2,30o fathoms ("Challenger") ; but most of them live in shallow littoral waters. The Chitons are mainly herbivorous and browse on the Algae that adhere to rocks. The members of the Neomeniidae which live on corals and hydroids feed on those organisms and are thus car nivorous. Free-living Neomeniomorpha such as Proparamenia are probably more aggressive and active in their feeding-habits. Chaetoderma and its allies feed on minute organisms such as Protozoa, etc.
The lack of satisfactory evidence as to the development of the Solenogastres makes it difficult to assess their relationships. The structural peculiarities on which Thiele, Odhner and others have relied in removing these animals from the mollusc lead us to be lieve that their affinity with the Chitons may not be so close as was formerly believed. But a critical test of their relationships can only be provided by a study of their development.
For the relationship of the Amphineura to the other groups of Mollusca, see MOLLUSCA.
BIBLIOGRAPHY.---P. Pelseneer, "The Mollusca" in Lankester's TreaBibliography.---P. Pelseneer, "The Mollusca" in Lankester's Trea- tise on Zoology (19o5) ; O. Hammersten and J. Runnstrom, Zoolog. Jahrbuch (1924) ; K. von Zittel, Grundziige der Pali1ozoologie (1924) ; J. Thiele, "Solenogastres" and "Loricata" in Kiikenthal and Krumbach's Handbuch der Zoologie (1925) ; E. Ashby, Rept. Australas. Assocn. Adv. Science z7 (1926). (G. C. R.)