When adhesion takes place between the pore-plates it is of course preceded by crowding, and this interferes with their equal development. Some which extend so far horizontally as to meet their fellows of the opposite side of the radius are called primary plates; others which are small and wedged in between the larger ones are called demi-plates. Systems of classification have been built up (chiefly by palaeontologists) in which great stress has been laid on how the primaries and secondaries enter into the constitution of the compound plate, but it does not seem to the present author as if this were at all a satisfactory basis for classification. All the pore-plates are primarily equivalent, and the question as to which are interfered with in their growth so as to become secondary is trivial. The so-called Arbacioid type consists of one primary with a secondary on each side; the Diadematoid type of three primaries, with occasionally a secondary between the aboral and the middle primary; and finally the Triplechinoid type of two primaries, with one or more secondaries between them.
Aristotle's Lantern.—Under this head we may consider the auriculae and gills as well as the jaws and teeth. In Cidaridae external gills appear to be absent, but from the lantern coelom large radial pouches project upwards into the general coelom cavity. These pouches are supposed to be respiratory, and are termed internal gills or Stewart's organs.[[488]] They co-exist with external gills in Echinothuriidae and in Diadematidae, though in the last family they are present only in a vestigial form, two being found in each radius. The auricular arch both in Cidaridae and in Arbaciidae is composed of two pillars which do not meet, but in the last-named family they are based, as in Echinidae, generally on the ambulacral plates, whereas in Cidaridae they arise from the interambulacral plates (the ambulacral plates being here very narrow). The epiphyses are absent in Cidaridae and Arbaciidae, and are imperfect in Diadematidae.
Spines.—These organs are extraordinarily variable, and usually differ very much in species of the same genus. In the vast majority of species there is a limited number of long spines called "primaries," amongst the bases of which a large number of much shorter "secondaries" are distributed. In Cidaridae the primaries are very long and thick and blunt at the ends, and the secondaries form small circles around their bases. The primaries in Cidaridae and the tips of the primaries in Arbaciidae and Echinothuriidae are covered with a special investment of extremely close, hard, calcareous matter very different from the loosely fenestrated material out of which the bodies of the spines of all species are composed. In Colobocentrotus and Heterocentrotus the primaries are very thick and triangular in section, whilst the secondaries on the aboral surface have expanded outer ends, which form a close-set pavement protecting the ectoderm from the shocks of the breakers. In Echinothuriidae the primaries are short and so delicate as to be termed silky.
Pedicellariae.—In Cidaridae only gemmiform and tridactyle pedicellariae are found. In the gemmiform the glands lie inside the grooved blades instead of outside as normally, and they are covered internally by ingrowths of calcareous matter from the edges. In Echinothuriidae only tridactyle and trifoliate are found in most species, but rudimentary gemmiform are found in one species and well-developed ophicephalous in another. In some species (Centrostephanus longispinosus) there are found gemmiform pedicellariae which have lost the jaws but retained the glands. These are termed "globiferae." Mortensen[[489]] uses minute details in the structure of the pedicellariae to discriminate species and even genera, but in this the present author is not prepared to follow him.
Tube-feet.—The tube-feet belonging to the aboral surface are pointed and devoid of a sucker in Diadematidae, Echinothuriidae, Arbaciidae, and Cidaridae;[[490]] in the last-named family those belonging to the oral surface have suckers, in the centre of which a pointed (sensory) prominence is to be noted.
The classification of the Endocyclica is by no means in a satisfactory condition, and different authorities have arrived at widely different results. Agassiz,[[491]] for instance, places the genera Echinus (the common British form) and Strongylocentrotus (the commonest American form) in different families. Bell,[[492]] on the other hand, considers them to be closely allied. Bell's system, based as it is on the development of the peristome, seems to the present author the most justifiable, for the peristome is undoubtedly a differentiation of the corona, which has been brought about by the manner in which the animal breathes and masticates, two functions of prime importance. The periproct is also of importance, representing as it does the whole aboral surface of the Starfish, and so are to a less extent the arrangements of the spines and of the tube-feet. Proceeding in this way, living Endocyclica can be divided into six families, which are briefly described below.
Fig. 235.—Oral view of dried and cleaned test of Cidaris. p, Pores for tube-feet arranged in single series; per, peristome with both ambulacral and interambulacral plates; t, tubercle of a large interambulacral spine.
Fam. 1. Cidaridae.—Endocyclica with a large peristome and a large periproct. The peristome is covered with a regular series of both ambulacral and interambulacral plates, the former pierced by tube-feet. No special buccal tube-feet and no external gills. The periproct is large, and is covered with irregular plates (Fig. 236, A). The lantern coelom is provided with large Stewart's organs.
The auriculae are incomplete and consist only of pillars arising from the interambulacral plates. The ambulacral pore-plates remain disunited, and the pores are arranged in a single vertical series; hence the ambulacra are very narrow. The interambulacral plates each bear one large primary spine surrounded by several circles of secondaries. No ophicephalous or trifoliate pedicellariae are to be found, and the gland of the gemmiform pedicellaria is placed inside the concavity of the blade.