KONOL. SV. VET. AKADEMIENS IIANDL. BAND. 19. N:0 7. 25 



III. THE AMBULACRAL SYSTEM. 



The peristome and the oesophageal opening in the young of Echini and Spatangi. The peculiar struc- 

 ture of these parts in Pourtalesin. The infra-frontal recess an incipient buccal cavity. The bivinin .ind its 

 abnormal structure. The spherids: their ventral position in Pourtalesia. The pedicels; their various forms in 

 the Spatangidfe; the evolution of the phyllodean pedicels; the pores: peripodia. Pourtalesia homneopodous: its 

 pedicels simple. 



Like one and another of his predecessors Linnaeus in some Spatangi overlooked 

 the front ambulacrum, and thus came to attribute to them only four ambulacra, an 

 error that was repeated by Lamarck, Cuvier, Blainville and others. In all Echi- 

 noids there are five ambulacra, or radii, the frontal one in many irregular forms differ- 

 ing more or less from the rest, and sometimes being but faintly marked. 



The young Echinoid, when on the point of assuming its final form, enveloped 

 in the larval membraneous covering, which is closed all around and destitute of any 

 opening, oral or anal, for a while floats beneath the surface of the sea, until, having 

 begun to take food, it sinks, and commences the mode of life of the adult. A minute 

 Echinus^), PL XIX, fig. 232, 233, genus unknown, 0,(i mm. in diameter, astomous and 

 aproctic, presents a spheroidal form and a smooth, unbroken, richly pigmented 

 surface above as below, with the spines standing on their tubercles, and, on the ven- 

 tral side, the primary pedicels over their pores. Within the envelope the final skele- 

 ton is forming, ten paired calcified laminae, the rudiments of the five ambulacra, each 

 giving passage, through a simple pore, to the circulatory apparatus on its way into the 

 tubes of the pedicels. Outward of these ten laminae five others are seen, answering to the 

 interstices between their pairs, and thus apparently representing the future interradia. 

 The five primary ambulacra here alone constitute the peristome, and the open space 

 enclosed within them is the future circular stoma. With this stage Johannes Mueller 

 first made us acquainted many years ago^). The act of opening outward of the ex- 

 tremities of the intestinal tube still remains to be observed. 



In Abatus cavernosus, as in many cases where the young are abnormally reared 

 by the parent or kept within the maternal body, or else under exceptional conditions 

 the development is abridged''), and those phases are left out, in which, as in the great 

 majority of the Echinoidea, the young has to lead a free oceanic life, and to evolve 

 its future permanent structure under the protection of peculiar, transitory, perhaps 

 mimically diverting larval forms. As described above, the development of Abatus ends 

 with a stage analogous to that well-known in the Echinidie, a resting stage, astomous 



') Etudes p. 27, pi. XVII, fig. 149. -) .Fohannes Mueller, Metamorphose der Echinoderraen: I, Berl. 

 Abhandl. 1848, sep. p. 22, pi. VII; IV, ib. 18.52, scp. p. 22, pi. IX, fig. 3, 4; VII, ib. 185.5, sep. p. 

 22, pi. VIII, fig. 9 — 11. — Krohn, Miillers Archiv, 1851, p. 351. — Al. Agassiz, Embryology of Ecliino- 

 derms, Mem. Amcr. Academy, IX, 1864, p. 1 ; Revision Echin., p. 709, pi. IX, fig. 1 ; pi. X. — Brooks. 

 Handbook of Invertebrate Zoology, Boston, 1882, p. 99—129, fig. 43—77. •'') Compare VVyville Thomson, 

 Voyage of the Challenger, II, p. 229. 



K. Sv. Vet.-Akad. Ilandl. Bd. 19. N:o 7. 4 



