Similar pinnate feet proceed from the pores of the petala in Petalosticha, a group in which there are locomotive feet both with and without terminal discs. It is a rule that the feet within an area inclosed by a fasciole differ from those without it. Brown pigment corpuscles, apparently respiratory in nature and containing iron, are met with in the coelome, water-vascular and blood-vascular vessels, together with corpuscles of different colours. The buccal or tegumentary gills, which are characteristic of the Ectobrajzchiate Desmosticha, are ten arborescent hollow diverticula of the coelome; they pass outwards in the peristome, each through a notch between the peristomial ends of the ambulacra and interambulacra. In the Cidaridae or E?ito-branchiate Desmosticha five diverticula with lateral branches project radially from the jaw-chambers between the alveoli into the coelome. Their walls are supported as a rule by calcareous deposits, and their cavities perhaps communicate with the exterior. They have also been found in a Diadema 1.

The digestive tract in the Desmosticha and Clypeastroidea commences with a pharynx surrounded by a complex masticatory organ, termed in the former 'Aristotle's lantern.' This apparatus consists of an interradial portion - viz. five sharp-pointed teeth supported by five sockets or alveoli, each composed of a right and left half covered above by corresponding epiphyses, and of a radial portion - viz. the rotida and the radii. The alveoli are massive and placed horizontally in Clypeastroidea, vertically in Desmosticha. The apparatus is moved by special muscles of which some are connected to the auricles. The Palaeo-echinoidea possessed similar masticatory organs. There is an oesophagus, followed by an intestine ending in a rectum. The commencement of the intestine is marked by a small dilatation in Desmosticha, by a large caecum in Petalosticha. The oesophagus lies in the madreporic interradius: the intestine is arranged in two coils, an inferior and a superior. The former passes through radius III to IV, V, I and II (see Loven's formula, note, p. 555) into the madreporic interradius, where the latter commences and turns back through radius II, to I, V and IV. The rectum commences in the interradius between III and IV, and runs vertically or obliquely to the anus.

Mesenteric bands connect the coils to the test, and in Petalosticha to one another. In the Desmosticka and Petalosticha a tube - the siphon - arises from the posterior extremity of the oesophagus and lies closely applied to the inner margin of the intestine into which it opens again at or near the end of the inferior coil. It is the 'convoluted organ' so-called of Spatangus. A second siphon has been observed in some Petalosticha (Schizaster, Brissus, etc.), There is a special anal muscle. The position of the anus has been already pointed out. The pedicellariae have been observed removing the faeces from the apex of the shell.

1 Stewart, Tr. L. S. (2), i. 1879.

The generative glands are arborescent caeca, large when sexually mature in Desmosticha, opening by a single duct on the basals or in the interambulacra (viviparous Cidaridae; some Clypeastroided). They are typically five, but, as already stated, the number is often reduced. The ripe testes are milk-white, the ovaries yellowish-brown.

The free swimming larva is a Pluteus. It differs from the Ophiuroid Pluteus in the absence of lateral arms, and the presence of an antero-internal pair to which are added in some Petalosticha an antero-external pair and three processes from the post-anal area. Many Echinid Plutei have a pair of ciliated 'epaulettes' on each side behind the ciliated band from which they are developed originally. They are not present in the Pluteus of Dorocidaris.

The deep-sea Urchins are related to forms prevalent in the Chalk. The Palaeo-echinoidea appear in the Lower Silurian strata. The oldest exocyclic forms are found in the Lias. Typical Clypeastroidea occur in the Upper Chalk, small however in size, and the larger genera develope from the Miocene to the present time. Typical Spatangideae appear in the Lower Chalk; the older forms (Collyrites, Clypeus) in middle Jurassic strata.

The Echinoidea may be classified into

1. Palaeo-Echinoidea

With a variable number of meridional rows of inter-ambulacral and sometimes of ambulacral plates. The plates overlap one another, and the test therefore cannot have been firm. Aristotle's lantern present. Anus exocyclic only in Cystocidaris. Basal plates pierced by three to five, radials by two, pores. Spines small as a rule, and restricted to interambulacra.

2. Desmosticha (Pegulares, Endocyclica)

Of spheroidal or flattened circular form. All ambulacra and interambulacra of equal length. Anus endocyclic. Mouth central. Complex manducatory organs (Aristotle's lantern). Subdivided into

(A) Entobranchiata

Cidaridae: devoid of external gills. Auricular arch not complete and interradial. Ambulacral and interambulacral plates continued on to peristome; pores in straight rows; pore plates primary and subequal.

(B) Ectobranchiata

External gills present. Auriculae extend over the radii. Interambulacral plates not continued on to peristome. (Saleniidae, Echinothuridae, Arbaciadae, Diade?natidae, Echinidae.)

3. Clypeastroidea (= Exocyclica In Part)

Shield shaped, often flattened. Mouth central, with manducatory organs. Dorsal portion of five ambulacra petaloid. Ambulacra of great breadth. Anus exocyclic. (Clypeastridae, Scutel-lidae.)

4. Petalosticha (= Exocyclica In Part, Spatangoidea)

More or less heart-shaped. Mouth more or less excentric; no manducatory organs. Anus exocyclic. Ambulacra petaloid, and anterior one often unlike the other four. Subdivided into

(A) Cassidulideae

Oval, with mouth central or subcentral; no petaloid ambulacra or all petaloid. (Echinoneidae, Cassidulidae.)

(B) Spatangideae

Mouth excentric and transversely elongate; a labrum; four ambulacra, petaloid as a rule; fascioles often present. (Collyritidae, Anan-chytidae, Spatangidae.)

General anatomical account, Kohler, Annales Mus. Nat. Hist. Marseilles, i. 1883. See also A. Agassiz, 'Revision of Echini,' Harvard Mus. Catalogue, No. vii. pt. iv. or iii, 1872-74; Id. Challenger Reports, iii. 1881.

Test, Zittel's Palaeontologie, Abth, 1, Palaeozoologie, i. 1876-80.

Classification of Desmosticha, Bell, P. Z. S. 1881.

Cyanosoma urens, an Echinothurid, and its poison apparatus, Sarasin (C. F. and P. B.), 'Ueber einen Lederigel,' etc, Z. A. ix, 1886. Pourtalesia, Lov^n, Kongl. Sv. Ak. Handl. xix. No. 7, 1883.

Pedicellariae, cf. lit, p, 194, and note, p. 558, ante. Sphaeridia, Ayres, Q. J. M. xxvi. 1886.

Structure of teeth, Giesbrecht, M. J, vi. 1880.

Nervous system, Prouho, C. R., 102, 1886; Romanes, 'Jellyfish, Starfish, and Sea-urchins,' Internat. series, 1885, cap. x.; Id. and Ewart, Ph. Tr. 172, 1881, p. 835.

Compound eyes in an Urchin (Diadema ?), Sarasin (C. F. and P. B.), . A. viii. 1884.

Vascular system, points in dispute, Carpenter, Q. J. M. xxiv. 1884; xxv, 1885. Echinochrome or blood-colouring matter, MacMunn, Q. J. M. xxv. 1885. Pluteus of Dorocidaris, Prouho, C. R. 101, 1885.