Echinoidea of the Swedish South Polar Expedition echinoideaofswed00mort Year: 1910 Bd. VI: 4) THE ECHINOIDEA. S3 pairs of double pores have been developed, and at about 9 mm. length they are already distinctly petaloid, with 4—5 pairs of pores in each series. In the arrangement of the tubercles no regularity is seen; it should only be noticed that the rather prominent tubercle found on each of the 5 primary inter- ambulacral plates round the peristome remains distinct till a rather late stage; that on the labrum may even be distinct in grown specimens. The peristome is originally circular, t
Echinoidea of the Swedish South Polar Expedition echinoideaofswed00mort Year: 1910 Bd. VI: 4) THE ECHINOIDEA. S3 pairs of double pores have been developed, and at about 9 mm. length they are already distinctly petaloid, with 4—5 pairs of pores in each series. In the arrangement of the tubercles no regularity is seen; it should only be noticed that the rather prominent tubercle found on each of the 5 primary inter- ambulacral plates round the peristome remains distinct till a rather late stage; that on the labrum may even be distinct in grown specimens. The peristome is originally circular, the mouth opening being in its centre. As seen by the figures on PI. IX the change in the shape of the peristome is produced mainly by the first plate of the odd poste- rior interambulacrum, the labrum, growing forwards, gradually covering the posterior part of the peristome and, at length, also the mouth opening. — AGASSIZ (Pan. Deep Sea Ech. p. 212) points out that in the Fis- 23- APical sXstem oi Abates cavernosus, 18 mm. in length. '• young all the interambulacra are in contact with the peristome, ^though in older specimens (PI. 99. Fig. 8) the posterior lateral ambulacra are excluded from the actinostomes. Apart from the lapsus calami, »ambulacra» for interambulacra, it may be pointed out 1) that in the figure quoted only the right posterior interambulacrum is shut out from the peristome, and 2) that this is no constant feature of the species; it seems to be more commonly the case that all the interambulacra remain in contact with the peristome, though only very narrowly, as in the left posterior interambulacrum in the figure given by AGASSIZ (Op. cit. PI. 99. 8). The pores of the madreporite do not increase in number until a rather late stage (comp. Figs. 19—22); in specimens of about 8 mm. length there may as yet be only a single pore, though, on the other hand, the second pore may appear somewhat earlier. — The first indications of the genital pores have been
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