Fig. 246.—Interior of test of Hemiaster philippi, showing the genital organs and their ducts (only three are developed in this species). Slightly enlarged. (From Wyville Thomson.)
Fig. 247.—Dissection of Echinocardium cordatum. × 1. The oral part of the test has been removed. caec, Blind pouch of the stomach; gon, genital organ; int, intestine; oe, oesophagus; rect, rectum; siph, siphon; st, stomach; w.v.r, water-vascular ring.
The stone-canal is very short and soon opens into the axial sinus; it is widely separated from the pore-canals which traverse the madreporite. Communication between the two is effected by the long axial sinus. There are only four genital organs.
Heart-urchins vary amongst themselves chiefly in the following points, viz.:—(1) the shape and position of the peristome, (2) the characters of the "petals," and (3) the number and position of the fascioles.
Peristome.—In many genera this is pentagonal and central, and in these cases the interradii commence at the peristome with a single plate, which is often covered with a thick crowd of small spines and is termed a "bourrelet." The oral ends of the radii also often consist of a crowded series of narrow plates, looking something like a "petal," and termed a "phyllode." The five bourrelets and five phyllodes constitute a flower-like figure termed a "floscelle."
Ambulacra.—In Echinoneus all five are alike and are provided with similar tube-feet, which are respiratory but possess suckers. The ambulacra are not grooved, and the petaloid arrangement of the pores is hardly marked; but in Cassidulus, Pourtalesia, and many other genera the five petals are well marked, though they are all similar to one another.
Fascioles.—These structures are often entirely absent; the sub-anal one alone is present in Spatangus. In Eupatagus a peripetalous one is added. This surrounds all the "petals," and has obviously the function of sweeping fresh water over the respiratory tube-feet. In Echinocardium, as we have seen, there is an "internal fasciole" between the two anterior petals which has a similar function. In addition, this genus possesses an anal fasciole which surrounds the anus and sweeps away the faeces.
