(Text-fig. 2, figs. 1–4.)
The peristome is that area of the animal which extends from the adoral edge of the calcareous plates of the corona to the mouth. Some authors (Lang, 1896) describe the corona and the peristome as together constituting the perisome. It is a tough fibrous membrane in which numerous calcareous plates are imbedded. At the oral edge it is terminated in a thick, circular lip (L) surrounding the projecting tips of the five teeth. In each radius immediately below the lips are the five pairs of buccal tube-feet (BF). These will be described in the section dealing with the ambulacial system. Each one is situated on a small buccal ambulacral plate (BP) roughly triangular in shape (15 × 25 mm) which is perforated by the excurrent and incurrent canals of the tube-feet. Around the edges of the plates numerous small tubercles (TBP) are scattered for the articulation of pedicellariae, which are mainly of the ophicephalous type (OP) although a few trifoliate (TF) may be present among them.
The remainder of the peristome is relatively bare. Small calcareous plates (Text-fig. 2, Fig. 3) are scattered here and there over it, particularly in the areas corresponding to the ambulacra of the corona. Each plate bears several tubercles for articulation with the stalks of trifoliate pedicellanae, which are very abundant in this area, although their smallness of size and delicacy of structure do not render them very obvious. No pedicellariae of any other kind are present. In addition, the peristome contains a large number of small fenestrated plates (Text-fig. 2, Fig. 4) imbedded within it. Spines, such as are present on the peristome of Echinus, are completely absent in Evechinus.
The peristome forms the floor of the lantern coelom and as such, in each interradius, is continued at its adoral edge, as a pair of branching outgrowths, or gills (Text-fig. 2, Fig. 1, G), described later in the section dealing with Aristotle's lantern and the lantern coelom. In repose the peristome lies quite flat, but when the lantern is protracted, the peristomial area becomes pushed out to assume a blunt conical shape. In these movements of the lantern, it is seen how necessary it is to have the area of the body immediately surrounding it completely flexible. If the mouth were surrounded by the hard, immovable plates covering the rest of the animal the lantern would be inoperable.
The diameter of the peristome increases with the size of the animal. In average size specimens it is usually between 20 to 25 mm in diameter, which is considered to be quite wide by Mortensen (1943), in comparison with other echinoids However, the peristome, expressed as a percentage of the horizontal diameter, decreases with age—e.g., in a specimen of 18 mm diameter it comprised 44% of the horizontal diameter, while in one of 98 mm diameter it comprised only 26%.
Some members of the Family Echinometridae—e.g., Zenocentrotus kellersi and species of Heterocentrotus—carry spines on the peristome in the same way as do species of Echinus (Mortensen, 1943). These are not present, however, in either Evechinus chloroticus or Heliocidarts erythrogramma The peristome of H. erythrogramma is very similar to that of Evechinus except that tridactyl pedrcellariae are also occasionally present scattered among the trifoliate. There are also fewer calcarcous plates. In width, comparative bareness of the buccal membrane, and the presence of delicate, fenestrated plates imbedded within it the peristome of Evechinus conforms to the general pattern exhibited by echinometrids.