MOUTH OF THE STAEFISH, 45 



tube, is the ocular tentacle. Ehrenberg discovered the presence of eyes 

 in Starfishes, but their true relations to this odd terminal tentacle was 

 first pointed out by Professor Agassiz, in his Homologies of Radiata. 



[The nature of this terminal tentacle in the young Starfish and all 

 young Echinoderms seems to have been entirely overlooked by all writers 

 who have described the eye of the Starfishes, which they have usually 

 represented as an organ totally unlike any other Echinodermal ap- 

 pendage. 



The Embryology of Echinoderms certainly shows most distinctly that 

 the eye of the Starfish is only a modified tentacle, an organ of sense, such 

 as we find at the base of the marginal tentacles of Acalephs.] 



Formation of the Mouth of the Starfish. — From the manner in which the 

 tentacles are formed by folds of the water-tube, it is plain that, in the 

 younger stages of the Echinoderm, the two ends of the circular tube must 

 remain disconnected ; the rapid accumulation of limestone particles on 

 the lower surfiice prevents us, however, from ascertaining this point. 

 Soon after the larva has disappeared, the whole actinal surface between 

 the pentagon of tentacles is covered by a membrane ; this membrane, in 

 the centre of which is placed the mouth, is the remnant of that part of 

 the larva situated in the groove between the anal and oral plastrons 

 {m, PI. VI. Fig. 12; PI. VII. Fig. 1). The mouth of the Starfish, how- 

 ever, is not in reality the mouth of the larva. During the shrinking of 

 the larva the long oesophagus has become shortened and contracted, 

 bringing the opening of the mouth of the larva to the level of the open- 

 ing of the oesophagus, which becomes eventually the true mouth of the 

 Starfish. 



Before the limestone particles have accumulated sufficiently to cover 

 the base of the radiating tubes, the mouth is movable, shifting its posi- 

 tion from one side to another indifferently (Pi. VI. Figs. 3, 7, 8, 12, m ; 

 PI. VII. Fig. 1), though by the time the deposit of limestone has formed 

 a small pentagon inside of the base of the radiating tubes, it has lost its 

 mobility. The water-pore (PL VI. Fig. 12, h\ or the madreporic body, 

 connects with the circular tube through a long, narrow tube, and is 

 placed on the actinal side in the angle between two rays; it is, as yet, 

 only a simple opening, protected by a thick funnel-shaped limestone pro- 

 jection (PI. VI. Fig. 12, I)). The young Starfish has no other anus than 

 that of the larva, which is placed on the very edge of the disk; but, 



