2o8 ECHINODERMATA 



DIAGRAM XI. 



ECHINODERMS. 



The figure named Aster, is a cross section of a starfish arm, showing 

 the external ectoderm, the calcareous plates (very dark), one of the small 

 spines (j/.), a skin-gill {sk.g.), the ambulacral ossicles (amb. oss.), the 

 radial nerve (».), *« radial blood-vessel {b.v.\ the radial water-vessel 

 {r.c), a tube-foot {t.f.), an ampulla [amp.), sections of the digestive cseca 

 {d.t:.), and of the reproductive organs («?/.)• (In P^t after Ludwig.) 



The figure named Antedon is a cross section (after Marshall), of one 

 of the arms of that Crihoid. It shows a section of a pinnule (p.), 

 the ambulacral nerve (k)> the antambulacral nerve {a. «.)> the am- 

 bulacral water-vessel (amb. c. ), the position of the reproductive strand 

 {rep.), the muscles of the arni (m.), and the ccelomic spaces. Below 

 Antedon is a vertical section of a Sea-urchin (after Huxley). At m. is the 

 mouth, «. is the nerve, c.c. the circular water-vessel around the top of 

 Aristotle's lantern, g. the gut, st. c. the stone-canal, r.c. a radial water- 

 vessel, amp. an ampulla, t.f. a tube-foot, p. a pedicellaria, sp. a spine, 

 a. the anus, m.t. the madreporic tubercle. 



To the right is a section of a Holothurian, showing the tentacles (t.), 

 the mouth (m.), the circumoral water- vessel (e.v.), the madreporic or 

 stone-canal {m.c), the reproductive organs [rep.), the intestine {int.), 

 the respiratory trees (resp. t. ), rising from the cloaca, and the five lines 

 of longitudinal muscles. 



At the foot of the page is a ripe ovum (l), a section of a blastosphere 

 (2), a section of a gastrula (3) with mesenchyme cells, and a section (4), 

 showing the origin of two ccelome pouches from the archenteron. 



Above are five larval forms : — Hoi. , the Auricularia of a Holothurian ; 

 Ast. , the Bipinnaria of a starfish ; Oph. , the Pluteus of an Ophiuroid ; 

 Ech., the Pluteus of an Echinoid ; Crin., the larva of Antedon, with 

 ciliated bands, and with calcareous plates forming within it. The 

 ciliated bands are indicated by the dark lines. (From Balfour.) 



