Fam. 1. Asteriidae.—Forcipulata in which the tube-feet are apparently arranged in four rows. Aboral skeleton a loose reticulum.
The general features of the family Asteriidae have been explained in the description of Asterias rubens (p. [432]). There are five well-marked species of the genus found on the British coasts. Of these A. glacialis is found chiefly in the south-western parts of the English Channel. It is a large Starfish of a purplish-grey colour, with large spines surrounded by cushions of pedicellariae arranged in one or two rows down each arm. A. muelleri resembles the foregoing species, but is of much smaller size, and is further distinguished by having straight pedicellariae in the neighbourhood of the ambulacral groove only. It is found on the east coast of Scotland, and carries its comparatively large eggs about with it until development is completed. A. rubens is the commonest species, and is found on both east and west coasts. Its colour is a bright orange, but varies to almost a straw colour. It is at once distinguished from the foregoing species by the spines of the dorsal surface, which are small and numerous, an irregular line of somewhat larger ones being sometimes seen down the centre of each arm. A. murrayi is a peculiar species restricted to the west coast of Scotland and Ireland. It has flattened arms, with vertical sides, and only three rows of small spines on the dorsal surface. It is of a violet colour. A. hispida is also a western species. It is a small Starfish with short stout arms; there are no straight pedicellariae, and only a few sharp spines on the dorsal surface.
On the eastern coast of North America there are several species of Asterias, of which the most noteworthy is the 6-rayed A. polaris of the Gulf of St. Lawrence. This species exhibits a marvellous range of colour-variation, ranging from bluish-violet through purple to red and straw-coloured. This variation seems to show that colour, as such, is of no importance to the animal, but probably depends on some compound of slightly varying composition which is being carried by the amoebocytes towards the exterior. On the Pacific coast there is a rich fauna of Starfish, among which we may mention as members of this family Asterias ochracea, a large violet species, so strong that it requires a severe wrench to detach it from the rock, and Pycnopodia with twenty-two arms.
Fam. 2. Heliasteridae.—Forcipulata allied to the Asteriidae, but with very numerous arms and double interradial septa. Heliaster.
Fam. 3. Zoroasteridae.—Forcipulata with the tube-feet in four rows at the base of the arm, in two rows at the tip. Aboral skeleton of almost contiguous plates bearing small spines or flattened scales. Zoroaster, Pholidaster.
Fam. 4. Stichasteridae.—Forcipulata with the tube-feet in four rows. Aboral skeleton of almost contiguous plates covered with granules. Stichaster, Tarsaster.
The Stichasteridae and Zoroasteridae have acquired a superficial resemblance to some of the long-armed Valvata, from which they are at once distinguished by their pedicellariae. It would be exceedingly interesting if more could be found out concerning the normal environment of these animals; it might then be possible to discover what is the cause of the assumption of this uniform mail of plates.
Fam. 5. Pedicellasteridae.—Forcipulata with two rows of tube-feet. The aboral skeleton bears projecting spines surrounded by cushions of straight pedicellariae. Pedicellaster, Coronaster.
Fam. 6. Brisingidae.—Forcipulata with numerous arms and only two rows of tube-feet. Aboral skeleton largely rudimentary and confined to the base of the arms. The small blunt spines are contained in sacs of skin covered with pedicellariae.
The Brisingidae, including Brisinga and Odinia, are a very remarkable family, chiefly on account of the smallness of the disc and of the extraordinary length of the arms. The arms have what we must consider to have been the primitive arrangement, since there is no lateral adhesion between them, and interbrachial septa are consequently entirely absent. The reduction of the skeleton is a very marked peculiarity and, like the tendency to the reduction of the skeleton of deep-sea fish, may stand in some relation to the great pressure under which the animals live.