Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/235

Rh be quoted as examples, the myology is much more com plicated, and anatomists have differed considerably in their respective views concerning the function of some of the muscles. They have been carefully described by Owen, Vogt, Hancock, Ciratiolet Woodward, and others, and more Figs. 23, 24. Lirgula anatina. 23. intciior of ventral valve ; 24, interior of dorsal valve (after King), g, umbonal muscular impressions (open valves) ; h, central muscles (close valves); 7, transmedial or sliding muscles; 6, parietal band; j, I , I, lateral muscles (j, anteriors; k, middles; /, outsiders), enabling the valves to move forward and backward on each other. recently by King, whose views seem to carry with them a greater degree of plausibility. Of the shell or valvular muscles he makes out five pairs and an odd one, and individualizes their re spective functions as fol lows : Three pairs are lateral, having their mem bers limited to the sides of the shell ; one pair are transmedians, each mem ber passing across the middle of the reverse side of the shell, while the odd muscle occupies the um bonal cavity. The cen tral and umbonal muscles effect the direct opening and closing of the shell, the laterals enable the valves to move forward and backward on each other, and the transme dians allow the similar extremities (the rostral) of the valves to turn from each other to the right or the left on an axis subcentrically situated, that is, the medio-trans- vcrse region of the dorsal valve. It was long a mat ter in discussion whether the animal could displace its valves sideways when about to open its shell, but this has been actually observed by Professors Semper and Morse, who saw the animal perform the operation. They mention that it is never done suddenly or by jerks, as the valves at are Fig. 25. Lingula anatina. Diagram showing the mug- cular system (after Hancock). The letters indicate the muscles as in figs. 23 and 24. A, dorsal, li, ventral valve; p, peduncle: c, heart; a, alimentary tube; t, anal aperture. first always pushed to one side several times and back again on each other, at the same time opening gradually in the transverse direction till they rest opposite to one another and widely apart. Those who have not seen the animal in life, or who did not believe in the possibility of the valves crossing each other with a slight obliquity, would not consent to appropriating any of its muscles to that purpose, and consequently attributed to all the lateral muscles the simple function of keeping the valves in an opposite position, or holding them adjusted. We have not only the observations of Semper and Morse, but the anatomical investigations of King, to confirm the sliding action or lateral divarication of the valves of lAnyula. [ missing text ] [ missing text ] In the Clistenterata, where no such sliding action of the valves was necessary or possible, no muscles for such an object were required, consequently none took rise from the lateral portions of the valves as in Linyida ; but in an extinct group, the Trimerdlidce, which seem to be some what intermediate in character between the Tretenterata and Clistenterata, have been found certain scars, which appear to have been produced by rudimentary lateral muscles, but it is doubtful (considering the shells are fur nished with teeth, though but rudely developed), whether such muscles enabled the valves, as in Lingida, to move forward and backward upon each other. We do not yet possess any reliable observations as to the manner in which Discina opens its valves, but Mr Barrett, who observed Crania in life, informs us that the valve opens by moving upon the straight hinge, without sliding the valve. There are muscles connected with other portions of the animal in both groups, such as the parietal muscles, strongly defined in the Tretenterata, and distinctive peculiarities of the peduncle, but the limited space at our disposal will not admit of entering upon further anatomical details.

The Brachiopoda all inhabit the sea, and are supposed to have attained their full growth in a single season. A vast amount of important and accurate information has been collected during the past ten years with respect to the geographical distribution of recent species, as well as to the marine depths they inhabit or prefer. This important knowledge is mainly due to the numerous well conducted and equipped dredging expeditions carried on by private individuals and by the Governments of the leading mari time states. It would not be possible to give here the names of all those naturalists who have contributed to this portion of our information, but we cannot pass over those of Edward Forbes, J. G. Jeffreys, W. B. Carpenter, W. H. Dall, W. Thomson, E. Suess, A. Adams, H. Cuming, &c. Previous to these investigations the data we possessed with respect to the habitat and ranges of depth were in most cases vague and unsatisfactory. It has been ascertained that the Brachiopoda are much localized, and usually occur in great numbers in their favourite haunts. Jeffreys does not believe that the habitat of any Invertebrate animal is affected by bathymetrical conditions, and that the same species will occur at various depths. We can say nothing certain with respect to the ranges of depth at which the extinct species lived, but some idea as to their probable depths can be surmised from a study of the recent species. As far as our present information will carry us the Tretenterata do not appear to have been found at a greater depth than 13GO to 2000 fathoms. Linyula abounds in particular haunts at about half the tide-mark, and partly buried in mud, or at depths varying from three or four inches from the surface of the sea to seventeen fathoms. Prof. Morse describes a species which he found in vast numbers in a sand shoal at low water ; the peduncle, six times the length of the shell, was partly encased in a sand tube (fig. 9). He observed likewise that this species (Lingolu pyramidata) had the power of moving over the 