CLASS II BRACHIOPODA 299 
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 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 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.— k BIG Boe: ; Magellania flavescens, Val. Median vertical 
: Liothyrina  vitrea, section, slightly enlarged. d, Spiral brachia ; 
The greater part of the Linn. sp. Recent. h, Fringed brachial margin ; pr, Cardinal process ; 
Bijerice) Gr Wrachial “ety heesiesinply 5 Simonteny anal 6 Month: #, Seno: « 
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, 4). 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 
v oa 
mined! WZ Wu, 
Wee. 
Seu 

