CLASS II BRACHIOPODA 363 



and form an irregulär mass or network. They appear to be absent in 

 Magellania, Terebratella and Lingula. The outer margins of the mantle are 

 thickened and set with numerous chitinous, simple 

 or barbed setae, sometimes of great length. 



The shell cavity is divided by a vertical mem- 

 branous wall, which is an extension of the mantle, 

 into two regions : a posterior, or visceral cavity, and 

 an anterior, or brachial- cavity. The posterior cavity 

 contains the principal viscera, the alimentary, cir- 

 culatory, nervous and muscular Systems. The an- 

 terior Chamber is occupied by the arms. 



Organs of the Visceral Cavity. — The membranous fig. 532. 



partition is pierced centrally by the oval or slit-like camarophoHa humUetonensis 

 mouth, from which the digestive tuhe extends back- Enjand. internal mouXshowing 

 wards as a simple or bent canal. In inarticulate JXr üSsonX ^*"'*^ ^'""^^^ 

 species, the alimentary canal is very long, makes 



several convolutions, and terminates in a well-defined anus, situated on one 

 side of the animal. In the Protremata and Telotremata the digestive tube 

 is shorter and much simpler than in the Atrernata and Neotremata. The 

 intestine makes a single convolution and terminates blindly in the living 

 representatives of these Orders, being surrounded by large hepatic lobes. 

 In many Paleozoic species it probably did not terminate blindly, since the 

 intestine passed through the hinge-plate by a central foramen. There is no 

 heart, circulation being apparently maintained by the cilia lining the vas- 

 cular sinuses. These sinuses pass into the perivisceral Chambers, and are 

 developed into vascular dilations at the back of the stomach and elsewhere. 

 These bodies are not contractile, and their function is unknown. Two 

 numerously branched vascular trunks diverge from the anterior portion of 

 the perivisceral Chambers, traversing the mantle in either valve to its 

 margins, and several others pass over the fleshy brachia for their entire 

 length. The nervous System consists of a circum-oesophageal ring on which 

 two supra-oesophageal ganglia are inserted. From the swellings of the 

 oesophageal ring (notably from that on the lower side), nerve fibres are 

 given off to the brachia, muscles, pedicle and the two lobes of the mantle. 

 In adult Brachiopods sense organs are not known with certainty ; but in 

 the embryos such are believed to be present. So far as is known the 

 sexes are always separate. The sexual organs in both male and female 

 are located essentially alike, and have a paired arrangement. Generally 

 they occupy the main trunks ^of the vascular sinuses, but may extend into 

 the visceral Chamber, or, in some of the inarticulate forms, may be restricted 

 to the latter. 



The Brachial Cavity. — The greater part of the anterior or brachial cavity 

 is occupied by the spirally enrolled labial appendages, the so-called arms, or 

 brachia. These are two in number, one at each side of the mouth, and are 

 of extremely delicate Constitution (Figs. 533, 534, and 531, ^). The tissue 

 of which they are composed is essentially cartilaginous, and is traversed by 

 several circulatory canals as well as by a groove. The' outer edges of the 

 brachia are fringed with long and movable cirri or tentacles, by means of which 

 currents are set up that conduct small food particles to the mouth. The 

 arms are frequently supported by a slender calcareous framework called 



