hynchonella_, where there are two pairs of kidneys, the
internal opening of the anterior pair is supported by the
gastroparietal band and that of the posterior pair by the ileoparietal
band. The latter pair alone persists in all other genera.

The kidneys or nephridia open internally by wide funnel-shaped
nephridiostomes and externally by small pores on each side of the
mouth near the base of the arms. Each is short, gently curved and
devoid of convolutions. They are lined by cells charged with a yellow
or brown pigment, and besides their excretory functions they act as
ducts through which the reproductive cells leave the body.

_Circulatory System._--The structures formerly regarded as
pseudohearts have been shown by Huxley to be nephridia; the true heart
was described and figured by A. Hancock, but has in many cases escaped
the observation of later zoologists. F. Blochmann in 1884, however,
observed this organ in the living animal in species of the following
genera:--_Terebratulina, Magellania_ [_Waldheimia_]_, Rhynchonella,
Megathyris_ (_Argiope_), _Lingula_, and _Crania_ (fig. 21). It
consists of a definite contractile sac or sacs lying on the dorsal
side of the alimentary canal near the oesophagus, and in preparations
of _Terebratulina_ made by quickly removing the viscera and examining
them in sea-water under a microscope, he was able to count the
pulsations, which followed one another at intervals of 30-40 seconds.

[Illustration: FIG. 23.--_Rhynchonella_ (_Hemithyris_) _psittacea._
Interior of dorsal valve, s, Sockets; b, dental plates; V, mouth;
de, labial appendage in its natural position; d, appendage extended
or unrolled.]

A vessel--the dorsal vessel--runs forward from the heart along the
dorsal surface of the oesophagus. This vessel is nothing but a split
between the right and left folds of the mesentery, and its cavity is
thus a remnant of the blastocoel. A similar primitive arrangement is
thought by F. Blochmann to obtain in the genital arteries. Anteriorly
the dorsal vessel splits into a right and a left half, which enter the
small arm-sinus and, running along it, give off a blind branch to each
tentacle (fig. 21). The right and left halves are connected ventrally
to the oesophagus by a short vessel which supplies these tentacles in
the immediate neighbourhood of the mouth. There is thus a vascular
ring around the oesophagus. The