Generation, Gemmation, Fission, Endogenous Cell Formation, Parthenogenesis, Sexual Reproduction, Hermaphroditism, Sexual Elements of Reproduction, Fecundation, Development of the Embryo, and Conjugation-Reproduction in the Protozoa: Investi- gations of Dallinger and Drysdale and others-Reproduction, &c., in the Porifera, Colenterata, Echinodermata, Trichoscolices, Anne- lida, Nematoscolices, Chatognatha, Onychophora, and Myriapoda— Reproduction, &c., in the Insecta: their Odours, Colours, Dances, and Music, as Agents in the Reproduction of the Insecta-Repro- duction in the Arachnida, Crustacea, Polyzoa, Brachiopoda, THE PHYSIOLOGY OF THE INVERTEBRATA. CHAPTER I. INTRODUCTION. ANIMAL physiology may be defined as that branch of biology which is concerned in the elucidation of the various functions which take place in the animal economy. It is a branch of study quite distinct from morphology, chorology, and ætiology; and as a separate branch of biological science we propose to treat it in the following pages. Researches undertaken to investigate accurately the proper physiological functions of the various organs and tissues of the Invertebrata were greatly needed; and it is only during the last few years that certain biological chemists-fully equipped with the necessary manipulative skill-have considerably advanced this important but much-neglected branch of biology. If one studies any particular organ from only one aspect, incomplete or erroneous conclusions are apt to be drawn. For instance, the vesicular tissue lying in the rectal loop in Ascidia, and in some species extending over the intestine, is well known to be renal in function. This vesicular tissue is a true kidney physiologically; morphologically it is another A matter, and depends upon one's definition of a true kidney. Embryologically these vesicles are the remains of a part of the original colon. As more attention has been paid to the morphology of the Invertebrata, it is not our object to speak of that branch of the subject further than is necessary; but in some cases the function of an organ or a tissue cannot be comprehended without referring to its anatomy. According to the great apostle of biological thought, “the actions of living matter are termed its functions; and these functions, varied as they are, may be reduced to three categories. They are either (1) Functions which affect the material composition of the body, and determine its mass, which is the balance of the processes of waste on the one hand, and those of assimilation on the other. Or (2) they are functions which subserve the process of reproduction, which is essentially the detachment of a part endowed with the power of developing into an independent whole. Or (3) they are functions in virtue of which one part of the body is able to exert a direct influence on another, and the body, by its parts or as a whole, becomes a source of molar motion. The first may be termed sustentative, the second generative, and the third correlative functions. In the lowest forms of life the functions which have been enumerated are seen in their simplest forms, and they are exerted indifferently, or nearly so, by all parts of the protoplasmic body; and the like is true of the functions of the body of even the highest organisms, so long as they are in the condition of the nucleated cell, which constitutes the starting-point of their development. But the first process in the development is the division of the germ into a number of morphological units or blastomeres, which eventually give rise to cells; and as each of these possesses the same physiological functions as the germ itself, it follows that each morphological * Prof. Huxley. unit is also a physiological unit, and the multi-cellular mass is strictly a compound organism, made up of a multitude of physiologically independent cells. The physiological activities manifested by the complex whole represent the sum, or rather the resultant, of the separate and independent physiological activities resident in each of the simpler constituents of that whole. "The morphological changes which the cells undergo in the course of the further development of the organism do not affect their individuality; and, notwithstanding the modification and confluence of its constituent cells, the adult organism, however complex, is still an aggregate of morphological units. Nor is it less an aggregate of physiological units, each of which retains its fundamental independence, though that independence becomes restricted in various ways. "Each cell, or that element of a tissue which proceeds from the modification of a cell, must needs retain its sustentative functions so long as it grows or maintains a condition of equilibrium; but the most completely metamorphosed cells show no trace of the generative function, and many exhibit no correlative functions. Contrariwise, those cells of the adult organism which are the unmetamorphosed derivatives of the germ exhibit all the primary functions, not only nourishing themselves and growing, but multiplying and frequently showing more or less marked movements." The cell theory, first ably worked out by Schwann, has led physiology, aided by chemical means, to scrutinise more profoundly the mechanism of the vital acts; it has taught it to refer them to their ultimate agents-that is, to the histological elements themselves, which vary in function and in form in complex beings, and which we must consider as playing a part in the mechanism of organised beings analogous to that of atoms in chemical molecules. In the lowest animals all functions are performed by all tissues: the sarcode of an amoeba assimilates, breathes, excretes, and reproduces-for no special part is set aside for the functions of digestion, of respiration, of excretion, of reproduction. There seems to be in the lowest Invertebrates a confusion of organic materials and functions. Many of the Protozoa are endowed with motility and sensibility, with a sort of instinct;* and yet, as far as we know at present, they are destitute of muscular and nervous elements. Possibly the sarcode is the rudiment, still undivided, of muscular fibre. But as we ascend gradually from lower to higher forms the differentiation becomes more marked, and we find particular parts of the body reserved for special actions. But this differentiation passes through various stages before arriving at the most differentiated forms of animal life. As already stated, the single cell of the amoeba performs many functions; and even when an organ has arrived at such a stage that it is quite distinct, it may have a dual or triple function-as, for instance, the pentagonal pyloric sac of Uraster rubens (one of the Asteridea) has been proved to have a dual function.† It is a digestive gland as well as an excretory organ, separating the nitrogenous products of the waste of the tissues, &c., from the blood in the form of uric acid, which is to be found in the five pouches of that organ. In The Origin of Species (chapter vi.) Darwin mentions the fact that "numerous cases could be given among the lower animals of the same organ performing at the same time wholly distinct functions: thus, in the larva of the dragon-fly . . . . the alimentary canal respires, digests, and excretes." But as we pass from the lower to the higher forms of animal life the various organs have special functions assigned them. This rule not only applies to the physiological functions of various organs, but also to their ana * See Binet's Psychic Life of Micro-Organisms. + See Dr. A. B. Griffiths' papers in the Proceedings of Royal Society of London, vol. 44, p. 325; and the Proceedings of Royal Society of Edinburgh, vol. 15, p. 11I. |