4 . 
CTIETACEOTJS ERACIITOPOBA 
secretes at its margin t.lic shell. This latter is very variable in structure; very com- 
monly it is porous and i>unctated, or fibrous and externally foliaceous. The former 
structure may be said to be typified by the fanaily Terehratulid/e, and the latter 
by the li jiynciionellidjR. The structural characters of the shell in different families 
have been elaborately treated by Prof. Carpenter in bis paper on the intimate 
structure of the shell of Bracbio])oda, included in the introductory part of the first 
volume of Mr. Davidson’s “Monograph of British fossil Bracbio])oda.” 
The inner layer of the mantle is very soft, and jiartly of reticulated structure. 
ToAvards the margins the two layers are generally more or less intimately connected, 
and provided on the inner side with long ebitinous or horny setae. Between the two 
layers is a net work of a highly developed vascular system, Avbicb is not for the 
purpose of respiration, as usually supposed, though it may assist in it. However, 
it is certain that it stands in intimate connection Avitb the development of the 
ova ; it leaves often strong impressions on the inner sides of the valves. 
The mouth is situated nearly centrally and at the anterior end of the body 
between the arms, being at their base often connected by a kind of velum. The 
alimentary canal is turned neurally, and is of A'aried length, rarely of the same 
thickness ; usually at a short distance from the mouth it is enlarged into a kind of 
stomach ; it terminates cither blindly or with a small opening behind the adductor 
muscle, and near the hinge-plate of the dorsal valve, either centrally or laterally. 
The liver is usually largely developed, consisting of numerous greenish blind 
sacs, as in other more highly organised IMollusca ; it envelopes the greater part of 
the alimentary canal from the mouth to its end. 
The centre of the circulation is a h&art, lying free in the cavity of the body 
above the stomach. It receives a strong vein anteriorly, and sends off two prin- 
cipal aorta branches, one to the mantle and the genital organs, and the other to the 
digestive organs and the peduncle. 
The usually spirally twisted arms, must he looked upon as true respiratory 
organs, although they besides appear to perform the function of supplying food to 
the mouth by producing a strong current of water. For this purpose they possess 
in the middle of the internal side a deep groove, and laterally one or more scries 
of very fine and long fillets. The mantle, w-hich is usually regarded as the organ of 
respiration,, does not perform this function, though it may assist in supjjlying a 
change in the Avater, but it does not possess the necessary vessels to conA^ey the blood 
to the heart. 
All or most of the recent Brachiopoda that have been examined appear to be 
hermaphrodites. The nervous system has its centre in a complete ring surrounding 
the oesophagus, and sending off branches to all parts of the body. 
As to the muscular system I need not specially mention the A^arious thin 
muscles and muscular loands which are required for keeping the digestive and other 
organs in their proper relative positions, and aa'IucIi are noccs.sary for their func- 
tions. Only four sets of muscles arc of general impoi’tancc; 1st, the addiwlor, 
