Fig. 200.—Aboral view of Archaster bifrons. × ¾. (From Wyville Thomson.)
Fam. 2. Astropectinidae.—Paxillosa which have lost the anus, but which possess neither aboral protuberance nor interradial grooves. The marginal plates are thick, covered with spinules and placed horizontally. The tube-feet have no suckers.
This family is the only one of the order which occurs in British waters, where it is represented by two genera, Astropecten and Luidia. In Astropecten the inferior marginal plate is in immediate contact with the adambulacral, whilst in Luidia it is separated from it by a small intermediate plate.
Fig. 201.—Oral view of Archaster bifrons. × ¾. (From Wyville Thomson.)
Astropecten irregularis is a very common species on the coast of Britain, and a study of its habits when in captivity has thrown a great deal of light on many obscure points in the anatomy of the Paxillosa. Owing to the loss of suckers it is unable to climb over rocks and stones like the ordinary species, but it runs over the surface of the hard sand in which it lives by means of its pointed tube-feet. The arms are highly muscular, and the animal when laid on its back rights itself by throwing the arms upwards and gradually overbalancing itself. The loss of suckers has also rendered Astropecten and its allies incapable of feeding in the manner described in the case of Asterias rubens. They are unable forcibly to open the valves of shell-fish, and the only resource left to them is to swallow their prey whole. The mouth is consequently wide, and the unfortunate victims, once inside the stomach, are compelled by suffocation to open sooner or later, when they are digested.[[456]]
Fig. 202.—Oral view of Psilaster acuminatus. × 4⁄3. adamb, Adambulacral spines; pax, paxillae; pod, pointed tube-feet devoid of sucker. (After Sladen.)
Many interesting experiments have been made on Astropecten by Preyer and other investigators, but one important fact[[457]] has escaped their notice, that Astropecten, when at rest, lies buried in the sand, whilst the centre of the aboral surface is raised into a cone which projects above the surface. On the sides of this cone the few papulae which this species possesses are distributed. This raising of the aboral surface is obviously an expedient to facilitate respiration. It loosens the sand over the region of the papulae, and thus allows the water to have access to them. We can thus understand how the restriction of the papulae to the dorsal surface, so characteristic of the Paxillosa, is not always as Sladen imagined, a primitive characteristic, but often an adaptation to the burrowing habits which in all probability are characteristic of the whole order. In both Luidia and Astropecten Cuénot has described short spines covered with cilia in the interspaces between the marginal plates, these also subserve respiration by drawing a current of water over the gills. Psilaster (Fig. 202).