CLASS II 



BRACHIOPODA 



and an anterior, or brachial cavity. The posterior cavity contains the 

 principal viscera, the alimentary, circulatory, nervous, and muscular systems. 

 The anterior chamber is occupied by the arms. 



Organs of the visceral cavity. The membranous partition is pierced cen- 

 trally by the oval or slit-like mouth, from which the digestive tube extends 

 backwards as a simple or bent canal. In inarticulate 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 

 Atremata and Neotremata. The intestine makes a single convolution and 

 terminates blindly in the living representatives of these orders, being sur- 

 rounded by large hepatic lobes. In many Palaeozoic 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 main- 

 tained by the cilia lining the vascular sinuses. These sinuses pass_inj:o 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 swell- 

 ings 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 ex- 

 tend into the visceral 

 chamber, or in some of 

 the inarticulate forms, 

 may be restricted to the 

 latter. 



The brachial cavity. Fl0 ' m ' 



rp, r .LI. Liothyrina vitrea, 



I he greater part ot the Linn. sp. Recent, 

 anterior, or brachial ^* U& simply 

 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. 496, 497, and 494, A). 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 



FIG. 497. 



Magellania fiavescens, Val. Median vertical 

 section, slightly enlarged, rf, Spiral brachia ; 

 h, Fringed brachial margin ; pr, Cardinal process ; 

 2, Alimentary canal ; v, Mouth ; ss, Septum ; a, 

 Adductors ; c, cf, Diductors (after Davidson). 



