Fig. 239.—View of peristome of Asthenosoma hystrix. amb, Ambulacral plates on the lower edge of the corona; inter, lower plates of the interambulacral area. (From Wyville Thomson.)
Fam. 3. Saleniidae.—Endocyclica with a large peristome and periproct. The peristome is covered with thin, scattered, irregular plates. There are five pairs of special buccal tube-feet, each supported by a special plate, and there are external gills. The periproct is excavated in the side of a large central pentagonal plate. It is covered with fifteen or twenty plates.[[496]] The ambulacral plates are separate as in the Cidaridae, but occasionally adhere in pairs near the peristome. The interambulacral plates also, as in Cidaridae, each bear one large primary spine surrounded by a circle of secondaries. A few deep-water forms belong to this family, the type genus Salenia (Fig. 240) being the best known. None occur in the British area. Superficially they resemble the Cidaridae, but in reality they are widely separated by the essentially modern character of the peristome.
Fig. 240.—Dried and cleaned shell of Salenia varispina, showing periproct covered by one large plate. × 4. (From Wyville Thomson.)
Fam. 4. Arbaciidae.—Endocyclica with a peristome on which, as in Saleniidae, there are only ten prominent plates perforated by the buccal tube-feet, and besides these thin irregular plates; external gills are present, and the auricles consist of incomplete arches springing from the ambulacral plates. The periproct is covered by four valve-like plates. The ambulacral pore-plates are separate near the periproct, but near the peristome unite on the "Arbacioid" pattern (v. p. [531]) to form secondary plates. The interambulacral plates each carry several spines. No representatives of this remarkable family are known in British waters, but Arbacia is found both on the east coast of North America and in the Mediterranean. It is distinguished by its conical test. All the upper tube-feet are devoid of a sucker; only those on the oral surface are used for locomotion.
Uexküll has studied the Mediterranean species, and has shown that the spines converge no matter how strong the stimulus may be, and so are incapable of aiding in locomotion; also that the ectoderm is devoid of ciliation, and hence the faecal matter which falls on the surface of the animal is not, as in other genera, allowed to fall off by the divergence of the spines nor swept off by the action of the cilia. In its natural habitat the wash of the ripples on the shore cleanses the animal. In captivity it is liable to suffocate itself.
Fam. 5. Diadematidae.—Endocyclica with a peristome similar to that of the Arbaciidae and the Saleniidae. External gills present and ten buccal tube-feet. Periproct small, covered with numerous small plates. The auricles form complete arches arising from the ambulacral region. Aristotle's lantern is provided with rudimentary Stewart's organs. The ambulacral pore-plates are separated at the apex, but unite orally in "Diadematoid" fashion (p. [531]) to form compound plates. The interambulacral plates bear numerous primaries. The aboral tube-feet are pointed, having lost their suckers.
This family is represented (according to Agassiz) at the present day by seven genera, none of which are found in British waters, though one (Centrostephanus) enters the Mediterranean. C. longispinus[[497]] was investigated by Uexküll and found to be distinguished by its sensitiveness to light and shade, and by the quickness of its movements, which were mainly carried out by its long spines. The family resembles the Arbaciidae in the pointed aboral tube-feet, but in the complete auriculae it resembles the next family.
Fam. 6. Echinidae.—Endocyclica with peristome and periproct as in the preceding family. External gills and buccal tube-feet present, but Stewart's organs totally absent. Ambulacral plates combined on the "Triplechinoid" plan (p. [531]) to form secondary plates. Interambulacral plates with numerous tubercles. All the tube-feet have suckers.
This family contains by far the larger number of living genera. It is divided into two sub-families, viz.:—