Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/233

Rh The mouth conducts by a narrow oesophagus to a simple stomach which is surrounded by a large granulated liver. Owen s &quot;hearts&quot; have been found to be oviducts, while the true heart consists of a pyriform vesicle appended to the dorsal surface of the stomach. The digestive organs and viscera, as well as the muscles, which take up only a small place in the neighbourhood of the beak, are separated from the great anterior cavity, and protected by a strong membrane in the centre of which the mouth is situated. The nervous system consists of a principal ganglion of no great size.

Both valves are lined by a delicate membrane termed the pallium or mantle ; it secretes the shell, and is generally fringed with horny bristles or sette. It is composed of an outer and inner layer, between which are situated the blood channels or laeunes ; in fact, all the internal parts of the shell are lined by the inner layer of the mantle, with the exception of the spots where the muscles attach themselves to the shell. The outer layer lines closely the inner sur face of the valves to which it adheres, and in those species in which the shell is traversed by canals there exist, on the surface of the mantle facing the inner surface of the valves, corresponding short cylindrical membranous pro jections or laeunes, which insert themselves into the small tubular orifices that traverse the shell. The csecal prolongations do not exist in those genera, such as Rhyn- chonella, where the shell is deprived of tubular perforations. The inner layer is rather thicker than the opposite one, and is covered with vibratile cilia, As stated by Nicholson and other anatomists, the blood channels form a remarkable system of more or less branched tubes, anastomosing with one another, and ending in the coacal extremities. This, which has been termed by Huxley the arterial system, com municates with the pcrivisceral cavity by means of two or four organs, which are called pseudo-hearts, and which were at one time supposed to be true hearts. Each pseudo- heart is divided into a narrow, elongated external portion (the so-called &quot;ventricle&quot;), which communicates, as Hancock has proved, by a small apicial aperture with the pallial cavity ; and a broad, funnel-shaped so-called &quot; auricle,&quot; communicating on the one hand by a constricted neck with the so-called &quot; ventricle,&quot; and on the other by a wide patent mouth with a chamber which occupies most of the cavity of the body proper, and sends more or less branched diverticula into the pallial lobes (Huxley). The channels vary in thoir dispositions and details in different genera, and as they project to some small extent, leave corresponding indentations on the inner surface of the shell, so that their shape and directions can very often be traced on fossil and extinct genera as well as if the animal was still in life ; this may be seen in the numerous illustrations appended to Davidson s and other authors works treating of fossil Brachiopoda. There are usually four principal arterial trunks in each lobo of the mantle; the two central ones run direct to the front, near to which they bifurcate, while the outer ones give off at intervals on the side facing the lateral margin of the valves a series of branches which bifurcate several times. It has been observed by Hancock that the inner lamina of the mantle, and more particularly that portion of it forming the floor of the great pallial sinuses, will undoubtedly assist in purifying the blood. In 1854, in his review of Davidson s great work on British fossil Brachiopoda, Oscar Schmidt called atten tion to an important anatomical omission, namely, the existence of a vast number of microscopic, flattened, cal careous, denticulated plates or spiculne on certain parts of the surface of the mantle, and destined, no doubt, to stiffen and protect the portions that contain them ; and it was, moreover, observed by Hancock, and afterwards by Des- longchamps, that these calcareous plates are not to be found equally distributed over all the surface of the mantle, but only in those portions in connection with the great laeunes or veins, the labial appendages, and the perivisccral cavity. These spiculoe do not appear to be present in every species, and are totally absent in Liiujula, llhynckoneUa, and others. Deslougchamps observes that if we examine the genera Kraussina, Terebratida, Tcrcbra- tulina, Megerlia, and Platydia, we have a series wherein the number and consistence uf the calcareous portions increase in a very rapid manner, and that the spicules lie over each other several times, leading the observers by insen sible degrees to Tkecidimn, in which the spicuke arc soldered together, and incrust the mantle to such an extent that it is no longer distinct from the shell itself. [ missing text ] [ missing text ]

The brachial ap- vf^Tv Brachial or pondages are a pair ^^^jlSl^V^^ labial ap- of singular organs ._ ^^^KfiJPlkT^L pondages, eminently charac teristic of the Bra chiopoda ; they often are more cor rectly termed labial appendages on ac count of each mem ber being a prolon gation of the lateral portion of the lips or margin of the mouth. The Lamel- libranchs or Con- chifera have analo gous appendages, Fig. 19. but Very much less Waldheimia.flavescens. Interior of dorsal valve, to sliow rloT/plrmo/l TVimr the position of the labial appendages. (A portion of eveiupeu. ^ J-u^,y tno fringe of cirri has been removed to show the assume different brachial membrane and a portion of the spiral cx- i Tr,. tremities of the arms.) shapes in duierent genera, and are supported, or otherwise, by the more or less complicated skeleton already described. The labial appendages, whatever may be the shape and convolutions they may assume, i .,.. , fill the larger por tion of the cavity of the shell in front of the visceral chamber ; they are formed of a mem branous tube, fringed on one side with long flexible cirri, and occupy almost the whole of the pallial cavity, but were not capable of being protruded in those families in which they were folded back upon them selves and support ed by a calcareous skeleton, as in Waldheimia, Tere- bratella, Megerlia, etc. Barrett, who has examined the animal of Terebratulina captit scrpentis in life, states that it showed more of itself than Wald heimia, cranium, which might be supposed from the labial appendages being in the first very slightly supported by a small loop. Barrett observed, likewise, that it protrudes its cirri further, that the cirri on the reflected part of the brachial appendage are shorter than on the first part, and were Fig. 20. 1T(i!Jhtiniia Jlaresccns. Lonjrltuillnnl section with a portion of the animal, rf, A, brnchial appendages; a, adductor; jc, c, divarloator muscles; t, septum , mouth; i, extremity of alimentary tube. The peduncular muscles have been purposely omitted. 