Coelom, Gonads and Excretory Organs.—These are closely similar in their relations to those of the Neomeniomorpha. The chief difference is that the gonad or generative portion of the coelom is single and median, opening into the pericardium by a single posterior aperture. The excretory organs or coelomoducts arise from the posterior corners of the pericardium, run forwards and then backwards to open by separate apertures lateral to the gills (fig. 5, A). There are no accessory generative organs.
The heart and vascular system are similar to those of the Neomeniomorpha, the only important differences being that the ventricle is nearly free in the pericardial cavity, and that the latter is traversed by the retractor muscles of the gills.
Nervous System.—There are two closely connected cerebral ganglia, from which arise the usual two pairs of nerve cords. Pallial and pedal on each side are closer together than in the other groups, and posteriorly they unite into a supra-rectal cord provided with a median ganglionic enlargement (fig. 7, C). A small stomatogastric commissure bearing two small ganglia arises from the cerebral ganglia and surrounds the oesophagus.
The development is at present entirely unknown.
General Remarks on the Amphineura.
The most important theoretical question concerning the Amphineura is how far do they represent the original condition of the ancestral mollusc? That is to say, we have to inquire which of their structural features is primitive and which modified. Their bilateral symmetry is obviously to be regarded as primitive, and the nervous system shows an original condition from which that of the asymmetrical twisted Gastropods can be derived. But in many other features both external and internal the three principal divisions differ so much from one another that we have to consider in the case of each organ-system which condition is the more primitive. According to Paul Pelseneer the Polyplacophora are the most archaic, the Aplacophora being specialized in (1) the great reduction of the foot, (2) the disappearance of the shell (Cryploplax among the Polyplacophora showing both reductions in progress), (3) the disappearance of the radula. But it is a widely recognized principle of morphology that a much modified animal is by no means modified to the same degree in all its organs. A form which is primitive on the whole may show a more advanced stage of evolution in some particular system of organs than another animal which is on the whole more highly developed and specialized. Thus the independent metamerism of certain organs in the Chitons is not primitive but acquired within the group: e.g. the shell valves and the ctenidia. And although embryology seems to prove that the Neomeniomorphs are derived from forms with a series of shell-valves, nevertheless it seems probable that the calcareous spicules which alone are present in adult Aplacophora preceded the solid shell in evolution.
It is held by some morphologists that the mollusc body is unsegmented, and therefore is to be compared to a single segment of a Chaetopod or Arthropod. In this case there should be only one pair of coelomoducts in the adult, the pair of true nephridia which should also occur being represented by the larval nephridia. There should also be only a single coelom, or a pair of lateral coelomic cavities. On this view then the Aplacophora are more primitive than the Polyplacophora in the relations of coelom, gonad and coelomoducts; and the genital ducts of the Chitons have arisen either by metameric repetition within the group, or by the gradual loss of an original connexion between the generative sac and the renal tube, as in Lamellibranchs and Gastropods, the generative sac acquiring a separate duct and opening to the exterior on each side.
Literature.—A. Sedgwick, “On certain Points in the Anatomy of Chiton,” Proc. R. Soc. Lond. xxxiii., 1881; J. Blumrich, “Das Integument der Chitonen,” Zeitsch. f. wiss. Zool. lii., 1891; A.C. Haddon, “Report on the Polyplacophora,” Challenger Reports. Zool. pt. xliii., 1886; H.N. Moseley, “On the presence of Eyes in the Shells of certain Chitonidae, and on the structure of these Organs,” Quart. Journ. Mic. Sci. new ser. xxv., 1885; A.A.W. Hubrecht, “Proneomenia Sluiteri,” Nied. Arch. f. Zool. Suppl. 1., 1881; A. Kowalewsky and A.F. Marion, “Contr. à l’histoire des Solenogastres ou Aplacophores,” Ann. Mus. Marseille, Zool. iii., 1887; A. Kowalewsky, “Sur le genre Chaetoderma,” Arch. de zool. expér. (3) ix., 1901; P. Pelseneer, “Mollusca,” Treatise on Zoology, edited by E. Ray Lankester, pt. v., 1906; E. Ray Lankester, “Mollusca,” in the 9th ed. of this Encyclopaedia, to which this article is much indebted.
(J. T. C.)