Heart with two auricles; ventricle traversed by the rectum, except in the Helicinidae.
Immediately before arrest the heart may beat much faster than normally, though with extreme irregularity, and in the lower animals the auricles may be observed occasionally to miss a beat, as in poisoning by veratrine and colchicum.
1 The anatomical error in reference to the auricles of Reptiles and Batrachians on the part of Linnaeus is extremely interesting, since it shows to what an extent the most patent facts may escape the observation of even the greatest observers, and what an amount of repeated dissection and unprejudiced attention has been necessary before the structure of the commonest animals has become known.
Most free rotifers swim by the corona, aided by the ciliated auricles when present.
A, Notholca longispina, lorica only; b, Anuraea aculeata, like the former, a floating pelagic type (plankton proper); c, Synchaeta stylata; corona with accessory antennae and sensory styles; auricles for swimming - an actively swimming pelagic type (nekton); d, Pterodina patina, with bdelloid corona and retractile foot with terminal ciliated cup; e, Distyla gissensis partly extended; f, Rattulus tigris.
Illoricata, cuticle soft; ciliated exsertile auricles above the disk sometimes present.
A dorsal contractile heart, with symmetrical right and left auricles receiving aerated blood from the ctenidia and mantle skirt, is present, being unequally developed only in those few forms which are inequivalve.
And of two auricles which open into the ventricle by orifices protected by valves.
The ventricle and auricles of Anodonta lie in a pericardium which is clothed with a pavement endothelium (d, fig.
Previously to Hoek's discovery a brown-coloured investment of the auricles of the heart of the oyster had been supposed to represent the nephridia in a rudimentary state.
In addition to the characters given above, it may be noted that the mantle is provided with a hypobranchial gland on the outer side of each gill, the auricles are muscular, the kidneys are glandular through their whole length, the sexes are separate.
In Nautilus two pairs of auricles are present, corresponding with the two pairs of ctenidia.
Some blood, however, enters the auricles directly from the mantle, without passing through the ctenidia.
The heart is enclosed in the pericardium, and consists of a median elongated ventricle and a pair of lateral auricles, so that the structure somewhat resembles that in the Lamellibranchiata.
The openings of the auricles into the ventricle vary in different forms. In many of the lower forms (Lepidopleuridae, Mopalidae, Ischnochitonidae) the opening on each side is single and anterior.
In the true Chitonidae there are generally two apertures on each side, and in two species three or four, another instance of the tendency to metameric repetition in the group. The auricles are connected with one another posteriorly behind the ventricle.