MEM. M. C. Z., IX. No. 2. — ECHINODEEMATA. 35 



12-18. Echinus lividus. From Johannes Miiller, Vierte Abhandliuig (1850-51), PI. VI., VU. Quoted above. 



A. First pair of dorsal arms. b. Stomach. 



B. Second pair of dorsal arms. i'. Intestine. 



E. Second pair of oral arms. c. Anus. 



F. First pair of oral arms. d. Cord of vibratlle cilia. 

 a. Mouth. /. Vibratile epaulettes, 

 a'. Qisopliagus. 



12. Pluteus on the sixteenth day after artificial fecundation ; this Pluteus was lemarlcably slow in its development. 



13, 14. Pluteus on the eleventh day after artificial fecundation, but more advanced than the preceding stage. 



13, seen obliquely from the side ; x is the outline of the depression iu which the inoutli is placed. 14, seen 

 from the mouth side. 



15. Somewhat more advanced Pluteus, on the seventeenth day, seen obliquely from the dorsal side, shows the rel- 



ative position of the calcareous rods, of the arms, and of the cord of vibratile cilia. 



16. Somewhat older Pluteus, in which the arms have greatly lengthened, and there is a slight swelling of the 



vibratile cord at the base of the dorsal arms, denoting the position of tlie future vibratile epaulettes. 



17. Fully developed Pluteus, with vibratile epaulettes ; the young Echinus is well advanced. 



18. Stage in which the young Echinus has resorlied the greater part of the Pluteus ; only a small part of the oral 



extremity ami short pieces of the rods of the arms of the larva are left. The rudimentary embryonic spines, 

 X, are quite prominent ; the ambulacral suckers, y, are clearly seen ; one of the longer tentacles projects 

 beyond the disk, and a few pedunculated pedicellarise have made their appearance. 



19-27. Echiiwcyamiis pusillu$ ? 



19-21. From .Toliannes Miiller, Vierte Abhandlung (1850-51), PI. VIII. Quoted above. 



A. First pair of dorsal arms. a'. (Esophagus. 



F. First pair of oral arms. I>- Stomach. 



a. Mouth. b'- Intestine. 



19. Young Pluteus, seen from the ventral side. 



20. The same, seen obliquely in profile from the dorsal side. 



21. Somewhat older Pluteus, in which the second pair of oral arms is already formed. It is somewhat problemat- 



ical if the stages of figs. 19-21 are the younger stages of figs. 22-24. 



22-27. From Johannes Midler, Siebeute Abhandlung, 1855, PI. VIII. Quoted above. 



a. Mouth. d. Intestine. 



b. Qisophagus. e. Anus. 



c. Stomach. 



22. Young Pluteus, with two dorsal and two oral arms, seen obliquely, looking into the mouth cavity. 



23. Somewhat older Pluteus, seen from the dorsal side ; the second pair of oral arms has made its ap|iearance. 



24. Fully grown PUiteus ; the young Urchin is not yet far advanced. 



25. Pluteus in which the resorption of the anal extremity is well advanced, the young Urchin occupying the whole 



of the anal extremity of the Pluteus ; the oral extremity is still nearly unchanged. 



26. The young Urchin seen in profile, after the complete resorption and disappearance of the plutean aj)pendages. 



J '" iu diameter. The odd tentacles have already well-developed suckers. 



27. The same young Urchin as fig. 26, under compression, showing the five teeth, the limestone jilates of the 



actinostome, and the single row of embryonic spines placed round the ambitus. 



28. Echiimrachnius parmal From Alexander .^gassiz. Revision of the Echini, 1874, Part IV., p. 727, fig. 65. 



lllust. Cat. Mus. Comp. Zool., No. VII. Part IV., 1874. 



28. Well-advanced Pluteus, remarkable, like the problematic Echinocyarmis Pluteus, fig. 24, for its rounded anal 



extremity : a, mouth ; a', oesophagus ; d, digestive cavity ; i, intestine ; a n, anus. Seen from the mouth 

 side. 



29. Echinus acutus ? From Johannes Miiller, Ueber die Larven und die Metamorphose .... (1846), PI. V. 



Quoted above. 



29. Fully developed Pluteus, with rounded anal extremity, large vibratile epaulettes, and comparatively short arms. 



Seen from the mouth side. A, first paii- of dorsal arms ; B, second pair of doi-sal arms ; a, mouth ; o', cesoph- 

 agus ; A, digestive cavity ; d, cord of vibratile cilia ; e, calcareous rods of arms ; /, vibi'atile epaulettes. 



30. Echinus brevispinosus. From Johannes Miiller, Siebente Abhandlung, 1855, PI. I. Quoted above. 



30. Fully developed Pluteus, seen from the dorsal side. This Pluteus is remarkable for the short pair of anal 



arms developed at the posterior extremity. 



