8 HISTOLOGY [ANIMAL. not due, however, to the accumulation of chondrin the chemical principle of the cartilage of vertebrates in intercellular substance. But a true cartilage is met with in some of the higher molluscs (Cephalopoda}, in which there is a considerable amount of intercellular substance, and the only difference, as compared with ordinary car tilage of most vertebrates, is that the cells are much ramified (as in some fishes). Bone, or osseous connective tissue, as the word is under stood in vertebrate histology, is not met with anywhere amongst the Invertebrata, and this is less to be wondered at since it does not make its appearance even in some of the lowest of the Vertebrata. But hard structures of various kinds serve to supply the place of bone as a sustentacular tissue, and these may be developed either within the con nective tissue, so as to form an internal hard skeleton, or on the exterior of the body, so as to form an external skeleton or shell. When external the shell is an epithelial structure, or at least is produced by the formative activity of the epithelium which covers the surface of the body. An internal hard skeleton may either coexist with an external, or the one may be found to the exclusion of the other. In the coelenterates the internal skeleton when found is generally deposited in the jelly-like intercellular substance in the form of separate spicules, which subsequently are cemented together by a further deposit of calcareous matter into a continuous skeleton. In the sponges the calcareous spicules often project fromthe jelly into the externalmedium, but it is probable that they are covered by an extension of the superficial flattened cells which they seem to pierce; the separate calcareous spicules are in some cases united by cal careous matter, in others by horny substance, or the spicules may be altogether absent and the horny framework constitute the whole skeleton. There is no evidence to show that these calcareous and 7lG - iG.-section of the shdi of . . ., - . , , Echinus, partly decalcified by homy deposits are formed by the acid. (Leydig.) The connective rh r-Pft no-pn nf trip pplla r,f trm tissue bundles are seen on the airect agency or the ceils or the left . somc are cut transversely; jelly-like tiSSUe, On the COn- on the right they are obscured i . p , , i , , i -i by the calcareous globules (c). trary, the fact that the spicules Above and below is seen a layer make their appearance in the of epithelium (e, *). jelly-like substance which accumulates between the two primary layers before there is any trace of cells to be seen in it is a fact pointing in the opposite direction. In the echinids amongst Echinodermata the shell is formed by a dense deposit of calcareous substance (tig. 16) in the fibrous connective tissue of the integument, but is not of the nature of bone, as has been sometimes supposed. In other echinoderms the deposition is more scanty, and in some (Holothuria) it may merely take the form of isolated spicules, which often present curious shapes. The Muscular Tissues of Animals. In the Ver tebrata three kinds of muscular tissue are met with the plain or involuntary, the cross-striped or voluntary, and the cardiac or heart muscle. Undoubtedly the last-named is to be regarded as a transitional form between the other two, for it combines some of the characters of each. This is especially well seen in the lower vertebrates, in which the muscular fibres of the heart (fig. 17) FIG. 17. consist of long, tapering, uninuclear cells, in ^" reo " form resembling the plain contractile fibre-cells, Frog s heart - but differing from these and resembling the multinucleated voluntary muscular fibres in exhibiting distinct transverse striations. Although these three kinds of muscular tissue thus differ from ono another in this respect, they agree in one im portant character. Whether transversely striated or not, they all exhibit a distinct longitudinal striation of their substance, which is probably indicatory of a polarity which the protoplasm of the cell has assumed at the same time with the faculty of becoming rapidly shortened in the direc tion of its length and coincidently with the loss of the power of contracting in other directions. Moreover, this longi tudinal striation is generally associated with the property of double refraction, which is exhibited to a marked degree by all kinds of muscular tissue. The voluntary muscular fibres are those in which the protoplasm of the original cell has undergone most dif ferentiation. If we trace their development we find that they originate from mesodermic cells which become elon gated in one direction, the nucleus undergoing a corre sponding change in shape, and soon becoming multiplied; we next find the external layer of the protoplasm becoming altered and converted into muscular substance, which exhibits from the first both a longitudinal and a transverse striation. The change in ques tion gradually extends inwards, so as to involve more and more of the protoplasm. Up to this time we can distinguish (fig. 18) in the muscular fibre a medullary part composed of unaltered protoplasm, with nuclei, and a cortical part com posed of differentiated muscle-substance. Sub sequently the nuclei leave their central situa- I p% 1 f 8 j~" tion, and either become scattered through the veioping muscular substance or come to lie entirely at Sta^fa the surface. There is always a little of the mammal. unaltered protoplasm to be found with each nucleus. In the plain muscular fibres, and in the cardiac muscular fibres, the nucleus does not multiply, and it maintains its central situation. The differentiation of the cell-protoplasm into muscle-substance begins at the periphery and extends towards the centre in the cells which constitute the heart- muscle as in the voluntary muscle, and it is probable that the same is the case in the plain muscular cells. The muscular fibres of the Invertebrata very closely resemble those of vertebrates. In most cases the differentia tion of the muscular substance is not so complete as in the voluntary muscles of vertebrates and especially of mam mals, but there is a striking exception in the Artliropoda, and especially in insects, where in conformity with the greater muscu lar activity they possess we find far better marked structural features. On this account the muscles of insects have been espe cially carefully studied with a view to the eluci dation of the structure of muscle generally. With a sufficiently high power a voluntary muscular fibre of an in sect (fig. 19) is seen to be composed of an ex- F]G 19 ._ TJvlnK nmsde of W ati-r-b U i-ti e (p v m- ternalstructurelessmem- s ></"/<), highly magnified. *, sar- . . , colemma; , dim stripe; 6, bright stripe; c, brane,- tiiesarCOlemma, rows of dots in bright stripe, which are seen a central strand of to be the knobbed heads oU muscle rods. nucleated protoplasm, and a semi-fluid substance the proper muscular substance lying between these and form ing almost the whole of the fibre. This proper muscular substance is composed of a clear doubly-refracting material, in which are embedded a number of minute rod-shaped particles, which arc so arranged side by side and end to I! ll iii Piltjl
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