almost constantly in motion, and often seen to convey small particles to the channel at their base, and that the cirri are bent up when the brachial appendages are retracted, but are generally uncoiled and straightened when the shell is opened, before which the animal has often been observed to protrude a few of its cirri, and move them about to ascertain if any danger threatened. In Rhyiichonella, where the elongated spiral labial appendages are slightly supported only at their origin by two short calcareous processes, they can at the will of the animal, according to Owen and Morse, be unrolled and protruded to some distance beyond the margins of the valves, and when forcibly stretched out they are said to be more than four times the length of theshell,andtosupport tome 3000 cirri. We may mention, likewise, that Otto Frederick Miiller, having dredged from the Lake of Droe- Fig. 21. back in Norway, a Shynchonellaptittacea. Interior of dorsal valve. m j j 7 *, sockets; 6, dental plates; V, mouth; rfc, number 01 lerebratulce ] abia i appendage in its natural position; </, (probably belonging to appendage extended or unrolled. Rhynchondla psittacea), and placed them in a glass of water, he observed that they gracefully extended their spirally coiled brachial appendages. It must, however, remain for ever uncertain whether, in the extinct genera Spirifera, Atrypa, and others, in which the spirally coiled ileshy labial appendages were supported throughout their entire length by a calcified skeleton, the animal could protrude them beyond the margin of the valves. In some families Rhynchonellidce, Prodiictida?, and others these organs are spiral and separate; in many the separation is only at their extremities. There can be very little doubt that these elegant organs, provided as they are with cirri and cilia, are not only instrumental in conveying microscopic organisms to the mouth, which, as seen in fig. 19, is situ ated between the appendages at their origin, but are like wise subservient to the functions of respiration. Hancock observes that to prove that the brachial organs subserve the function of gills, as well as that of sustentation, it is only necessary to refer to the manner in which the blood circles round the labial appendages and is carried to the cirri, but more particularly to its circulating through these latter
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As the number and position of the muscles differ materially in the two great divisions into which the Brachiopoda have been grouped, and to some extent also in the different genera of which each division is composed, it may be desirable to treat this subject under two separate heads. Unfortunately almost every anatomist who has written on the muscles of the Brachiopoda has proposed different names for each muscle, and the confusion thence arising is much to be regretted. In the Clistenterata, of which the genus Terclratida may be taken as an example, five or six pairs of muscles are stated by Hancock, Gratiolet, and others, to be connected with the opening and closing of the valves, or with their attachment to or movements upon the peduncle. First of all, the adductors or occlusors consist of two muscles, which, bifurcating near the centre of the shell cavity, produce a large quadruple impression on the internal surface of the small valve (fig. 13, a, a), and a single divided one towards the centre of the large or ventral valve (fig. 12, a). The function of this pair of muscles is the closing of the valves. Gratiolet, who has likewise described with great minuteness the muscles of the Brachiopoda, informs us that those which close and open the valves were the only ones known to Pallas, but that he defined their position and functions clearly. The same was done by Blainville and Quenstedt, but the absence of good figures caused much uncertainty to prevail. This Fig. 22. WahVieimia flarescens. Diagram showing the muscular system (after Hancock) M. ventral, N, dorsal valve; /, loop; V, mouth; Z, extremity of ink-lit ine; o, adductor; c, divaricators ; c 1 , accessory divaricators; ft, ventral adjusters; 6, peduncular muscles; 6", dorsal adjusters; 1 , peduncle. deficiency was subsequently supplied by Hancock and Gratiolet s admirable illustrations. Two other pairs have been termed divaricators by Hancock, or cardinal muscles ("muscles diducteurs " of Gratiolet), and have for function the opening of the valves. The divaricators proper aro stated by Hancock to arise from the ventral valve, one on each side, a little in advance of and close to the adductors, and after rapidly diminishing in size become attached to the cardinal process, a space or prominence between the sockets in the dorsal valve. The accessory divaricators are, according to the same authority, a pair of small muscles which have their ends attached to the ventral valve, one on each side of the median line, a little behind the united basis of the adductors, and again to the extreme point of the cardinal process. Two pairs of muscles, apparently con nected with the peduncle and its limited movements, have been minutely described by Hancock as having one of their extremities attached to this organ. The dorsal adjusters are fixed to the ventral surface of the peduncle, and are again inserted into the hinge-plate in the smaller valve. The ventral adjusters are considered to pass from the inner extremity of the peduncle, and to become attached by one pair of their extremities to the ventral valve, one on each side of and a little behind the expanded base of the divaricators. The function of these muscles, according to the same authority, is not only that of erecting the shell, they serve also to attach the peduncle to the shell, and thus effect the steadying of it upon the peduncle. Gratiolet describes the peduncle with great care, and states it to be composed of two portions 1st, of a horny sheath formed of concentric epidermic layers, very analogous to that which Vogt has described in Lwgula ; and 2d, a fibrous stem enveloped by the sheath. This stem, composed of tendinous fibres, is fixed by its free extremity to different submarine objects ; the other extremity passes through the foramen, and is ended by a bulbous projection.
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Such is the general arrangement of the shell muscles in the division composing the articulated Brachiopoda, making allowance for certain unimportant modifications observable in the animals composing the different families and genera thereof. Owing to the strong and tight interlocking of the valves by the means of curved teeth and sockets, many species of Brachiopoda could open their valves but slightly. In some species, such as Thecidium, the animal could raise its dorsal valve at right angles to the plane of the ventral one (fig. 4).
