262 



NATURAL HISTORY. 



The interior of each valve of 



June to October, by imitating as far as possible their natural surroundings. They would often 

 protrude above the surface of the sand, and instantly jerk back when alarmed. On emptying the 

 sand from the bowl one day, great was my surprise to find that all of the Lingulse had covered the 

 bottom of the bowl with large irregular sand-tubes, cemented to the sides and bottom of the dish, the 

 tubes running over each other, and presenting precisely the appearance as that produced by Terabella 

 and allied forms when kept in dishes in this way." The peduncle is sometimes preserved in a fossil 

 state. 



shell is invariably lined by the pallium or mantle, a 

 delicate membrane which closely invests the body of 

 the animal occupant. It is divided into two lobes, 

 which are united only at the peduncle and extend to 

 the outer edges of the shell, where they are fringed with 

 bristles (Fig. 7, s) of variable thickness, and length. The 

 mantle is composed of two distinct layers : the outer 

 adheres closely to the valves of those genera in which 

 the shell substance is perforated by canals which are 

 T.-LIXGULA PYIUMIDATA. (After Morse.) partially filled with its minute projections. The under 



T, Peduncle; t, its Terminal end. encased in Sand tube; s, S.-tas layer, of a thinner, granulated textui'6, is clothed with 



vibratile cilia. The mantle secretes, nourishes, and 



repairs the shell substance, and is also concerned with the processes of respiration, the purification 

 and circulation of the blood. It also serves in some cases as a medium for the discharge of 

 the eggs, which apparently accumulate in the larger sinuses of its arterial system. In some genera, 

 a series of very minute calcareous plates stiffen the mantle and form a protection over the veins, the 

 breathing organs, and the perivisceral cavity as that small portion of the shell which is occupied by 

 the digestive organs is termed, to distinguish it from the larger space the pallial chamber filled 

 by the mantle. This separated from the former by a membranous wall clothed with cilia 

 contains only the brachial organs and mantle, and may be compared with the tentacular sheath of 

 the Moss-animal. (See p. 273.) 



The largely-developed fleshy brachial organs, so characteristic of the Brachiopoda, occupy a 

 considerable portion of the pallial cavity of the shell. Formerly regarded as uniting the functions 

 of respiration and locomotion, they gave the name of Brachiopoda to the group. But they are more 

 correctly designated the brachial appendages. They are often supported by the calcified loop-shaped 

 prolongations of the shell lip already alluded to (Fig. 5, B, I). They are now known to act as 

 accessory breathing and circulatory organs, and are also employed in collecting and directing towards 

 the mouth situated at their base currents of water ; and in eliminating therefrom the microscopic 

 organisms, consisting chiefly of minute plants, diatomacese and infusorian animalcules which con- 

 stitute the nourishment of the animal inhabiting the shell. Hancock describes these organs as 

 consisting of a flexible membranous tube, fringed on one side by a double row of hollow ten- 

 tacles or cirri (Fig. 8, i). Each of these is capable of separate movement, and is in turn clothed with 

 pliant hairs or cilia. These cilia draw the " minute nutrient particles " down into the groove or 

 gutter which lies at the base of the tube, which is also lined with cilia, and so the nourishment is 

 conveyed down its course to the mouth situated at the origin of the arms. This opening a 

 simple slit communicates with a comparatively short gullet, which leads into a more elongated 

 stomach. The alimentary canal, of variable length, ends somewhat abruptly in a blind sac in 

 the lower forms. In the higher, it is prolonged, turns upwards, and terminates in an ana) 

 orifice (g) between the right edges of the mantle nearest the dorsal valve. 



The liver (Fig. 8, r) of a greenish colour and very large is divided into two lobes, respectively 

 situated on each side of the alimentary canal. It communicates by two or three short ducts with 

 the stomach, and in the higher forms is flanked by the large-sized genital organs of both sexes. 

 Among the Clistenterates the genei-ative products developed and set free in the perivisceral cavity 

 are conveyed through trumpet-shaped a:id ciliated ducts into the pallial chamber, thence gaining 

 access to the sea-water. The pallial blood-channels lie between the two layers composing the mantle. 

 A series of canals running one into the other, and presumed to be vascular, permeate the membranous 



