378 



BliACHIOPODA.- 



-MOLLUSCA.- 



-Bkachiopoda. 



Class V.— BRACHIOPODA (Brachiopods.) 



TilK Brachiopoils are so closely allied to the con- 

 cliiferous mulliisks, that they have been considered by 

 many naturalists as forming only a simple division of 

 that extensive class. " In all essential points," says 

 I'rofessor Owen, "the BracMojmda closely corres- 

 pond with the acephalous Mollusca, and we consiiler 

 them as being intermediate to the Lamellihranchiate 

 and Tunicate orders ; not, however, possessing, as far 

 as they are at present known, a distinctive character of 

 sufficient importance to justify their being regarded as 

 a distinct class of nioUusks, but forming a separate 

 group of equal value with the Lamellihrinchiata." In 

 accordance, however, with the more modern systems 

 of classification, as propounded by Dr. Gray, Mr. Wood- 

 ward, and the Messrs. Adams, we treat them as a 

 separate class, the pecidiar structure of the animal en- 

 titling them to such distinction. Though this struc- 

 ture shows them to be comparatively low in the scale, 

 yet they are exceedingly interestiijg both to the physi- 

 ologist and the geologist. The number of recent or 

 living species is small, but in former periods of the 

 earth's surface it was very great, and the Brachiopods 

 at those times occupied tlie position now taken by the 

 Cunchifira. The shell of the Brachiopodons mollusca 

 is essentially bivalve. It is symmetrical but inequi- 

 valve, and is generally attached to submarine bodies 

 by a muscnlar peduncle which passes through a hole 

 in the beak. This in all probability is a byssus when 

 the animal is young, but becomes consolidated as it 

 increases in age, and forms at length a permanent 

 organ of attachment. The valves, instead of being 

 right and left as in the Cunchifera, are superior and 

 inferior. As tlie animal lies upon its back, the lower 

 valve covers the back of the animal, and therefore is, 

 with reference to the body, correctly called the dorsal 

 valve ; the upper, covering the ventral surface, receives 

 the name of ventral valve. The dorsal valve is the 

 smaller of the two in general, and is always free ; the 

 ventral valve, on the contrary, is usually the larger, 

 and has its umbo produced and perforated for the pas- 

 sage of its attaching peduncle. These valves are united 

 by a hinge without a. ligament, and are articulated by 

 two curved teeth, which aj'e developed from the margin 

 of the ventral valve and fit corresponding sockets in the 

 other. Internally the dorsal valve is fw-nished with 

 a peculiar shelly apparatus, called by some authors the 

 " apophysary system," and by others the " internal 

 skeleton, and by collectors the " carriage-spring." 

 ".The principal part of the internal skeleton," says 

 Professor Owen, "consists of a slender, flattened, cal- 

 careous loop, the extremities of which are attached to 

 the lateral elevated ridges of the hinge. The crura of 

 tlie loop diverge, but again approximate to each other, as 

 they advance for a greater or less distance towards the 

 opposite margin of the valve. The loop then suddenly 

 tiu-ns towards the perforate valve, and is bent back 

 upon itself for a greater or less extent in diflerent species. 

 .... The arches of the loop are so slender that, 



notwithstanding their calcareous nature they possess a 

 slight degree of elasticity and yield a little to pressure, 

 but for the same reason they readily- break off if the 

 experiment be not made with due caution." (See cut 

 annexed — fig. 230 — upper figure.) This extraordinary 

 apparatus is intended for the support of the " fringed 



structure of Bracii'unMjds. 



arms" of the animal, a struclure which is peculiar to 

 this class of molhisks, and which has obtained for them 

 tlieir name.* (See lower figure.) The mouth of the 

 Brachiopods is placed on the under part of the cavity 

 between the lobes of the mantle. Developed froiu the 

 sides of this mouth we see two long appendages, which 

 appear to be lateral prolongations of the lips, and cor- 

 respond with the lips and labial tentacles of the ordi- 

 nary bivalves. These have usually been called arms 

 — they are free or united by membrane, and are so 

 long that they require to be folded up. In most of the 

 siiecies they are twisted in a spiraJ form, and their 

 outer margins are fringed with longcirrhi or fiJamentB, 

 by means of which the anima] creates a current in the 

 water that brings objects of food within reach of the 

 mouth. Some conchologists imagine that tlie animal 

 has the power of protruding these oral arms beyond 

 the shell in search of food ; but this does not appear to 

 be the case, as from the fact of their being united by a 

 membrane, and supported by the internal skeleton 

 described above, they must evidently be confined to 

 one position. In some of the genera, however, we 

 are told that the ends of their arms are " endowed with 

 a degree of voluntary motion, the movement being 

 effected by the injection of a fluid into the hollow spiral 

 tube, by means of which the coils are separated." 



* From iSj«x'o', arm, and ^'"s, foot, these arms having be<'n 

 supposed to take the place of the feet in the preceding clasbcs. 



