liuACHIOPODA.- 



-MOLLUSCA. - 



-Ti:KEIUlATl"I.n>.E. 



379 



Another peciJiaiity ia the structure of the Brachio- 

 podous Mollusca is their respiratory system. True 

 gills are altogether wanting, and accordingl}', by some 

 of the earlier conchologists, these oral or fringed arms 

 were considered as subsidiary to respiration and the 

 aeration of the blood ; but Professor Owen, in his 

 masterly paper in the Zoological Journal, from which 

 we have already given an extract or two, shows that 

 these processes are carried on by the mantle. The 

 true branchial vessels, he says, are ramified in rich 

 profusion upon the inner surface of the lubes of this 

 part of the body, and consequently it forms the chief, 

 if not the sole respiratory organ. " In this profuse 

 distribution of vessels over a plain membraneous ex- 

 pansion," says the Professor, " we perceive the simplest 

 construction of the water-breathing organ, or branc/iia." 

 In some of the Brachiopods the ova are developed in 

 these large branchial veins, and in others they are 

 developed in vascular sinuses on each side of the body. 

 Nothing accurate is known respecting the development 

 of the young, but they are believed to be, in tlieir lirst 

 stage, free and able to swim about until they meet with 

 a suitable position. Some of them appear to attain 

 their full growth in a single season, and all, probably, 

 live many years after they have become adult. The 

 species are all marine, and are found attached to 

 branches of coral, hanging from the under side of 

 shelving rocks, or abiding in the cavities of other shells. 

 They are seldom found on clay beds ; but where flie 

 bottom consists of calcareous mud they appear to be 

 very abundant, mooring themselves to every hard sub- 

 stance on the sea bed, and clustering one upon the 

 other. — ( Woodward.) The Brachiopods enjoyagreater 



range both of climate and depth, as well as time, than 

 any other Mollusca known. They are found in tropical, 

 temperate, and arctic seas ; in shallow pools left by 

 the retiring tide, and at the greatest depths explored 

 by the dredges ; whilst, in time, they are found fossil 

 in the miocene tertiary, and in the oldest Devonian 

 strata. Indeed the oldest form of organic life at pre- 

 sent known, both in the Old and New world, is a 

 Limjula. At present there are only between seventy 

 and eighty recent species known, whilst of extinct 

 forms upwards of one thousand have been described. 



The class Bracukipoda is divided into two sub- 

 classes — I. Ancylopoda, and II. Helictopoda. Ia 

 the Ancylopoda, the oral arms are recurved; not 

 extensile, or only at the tip ; and are affixed to shelly 

 appendages on the disc of the dorsal valve. The 

 mantle is adherent to the shell, the substance of which 

 is pierced by numerous minute perforations. In the 

 Helictopoda the oral arms are elongate and regularly 

 spirally twisted when in repose. The mantle lobes 

 are merely applied to the inner surface of the shell, the 

 surface of which is not punctured. 



The sub-class Ancylopoda is divided again into two 

 orders — I. Ancylobraohia, and II. ChYPTonRACiiiA. 



The Ancylohrachia have the oral arms aflixed to 

 calcareous lamellae, forming a loop attached to tlie 

 hinge margin of the dorsal valve, and more or less pro- 

 minent in its cavity. The animal is generally attached 

 to marine bodies by a tendinous peduncle. The Cryj)- 

 tohrachia have the oral arms sunk into grooves in the 

 convex centre of the inner surface of the ventral valve. 

 The shells are generally attached by the outer surface 

 of the larger valve. 



Sub-class I.— ANCYLOPODA. 



Order I.— ANCYLOBRACHIA. 



This order contains only two families: — 



Family -TEREBRATULID^ [The Lamp-sJielh). 



The shells in this family (see Plate 11, fig. 2G, 

 Terehralula austmUs) are inequivalve, and usually 

 oral or suboircular. They are somewhat like a 

 Grecian lamp in form, and have therefore received 

 the general appellation of Lamp-shells. The valves 

 are minutely punctate, sometimes smooth, some- 

 times striated. The ventral valve is the larger, 

 and its beak is produced and truncated at the apex, 

 which is perforated. The foramen or hole is some- 

 times perfect, at other times it is a deep sinus, uncon- 

 nected at its lower margin. It is usually separated 

 from the hinge line by a triangular plate, called the 

 ddtidium, which is composed of two pieces. The two 

 teeth are placed in this valve, situated one on each side 

 of the deltidium. The dorsal, and smaller valve, has 

 a depressed beak, two sockets for the reception of the 



teeth of the ventral valve, and a prominent cardinal 

 process between them. It is also furnished with a 

 hinge plate provided with four cavities, and a central 

 ridge or sepitum. The internal skeleton (see fig. 204) is 

 attached to this valve, and consists of a slender shelly 

 loop attached in some genera at its origin to the hinge 

 plate, and furnished with oral processes ; in others to 

 the septum in the middle of the dorsal valve. The 

 animal is always attached by a pednncle, which passes 

 through the hole or deep sinus in the apex of the beak; 

 and tlie strongly cirrhated, looped, or contorted oral 

 arms (described above) are united throughout by mem- 

 brane, are folded upon themselves, and are only spiral 

 at their extremities. The valves are opened and closed 

 by adductor and retractor muscles, and the pedimde 

 is provided with two additional ones. The species, 

 though not numerous, have been distributed through- 

 out a number of genera, whose characters have been 

 taken chiefly from the structure of the internal skele- 

 ton. Tliey may all be arranged in two tribes. 



