Chap. II. MOUTH OF THE STARFISH. 37 



ing leading into the radiating canal contracts, the basal portion of the tentacle 

 swells, and it assumes a somewhat pear-shaped form, the swelling at the base 

 increases, principally on the oral side, and we soon trace in it an accumulation of 

 pigment cells (PI. VII. Fig. 6, e), which, by the time the other tentacles have 

 developed knobs, and equal in length the diameter of the arms, has become a 

 brilliant carmine spot (PI. VI. Fig. 12, e; PL VII. Fig. 6, e, and PL VIII. Fig. 5, e). 

 This odd tentacle, placed at the extremity of the radiating 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 Homolo- 

 gies of Radiata. 



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 surface 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, 

 PL VI. Fig. 12; PL VII. Fig. 1). The mouth of the Starfish, however, is not in 

 reality the mouth of the larva. During the shrinking of the larva, the long oesoph- 

 agus has becomed shortened and contracted, bringing the opening of the mouth of the 

 larva to the level of the opening 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 position from one side to an- 

 other indifferently (PL VI. Figs. 3, 7, 8, 12, m ; PL 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, b), 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, 

 protectecj by a thick funnel-shaped limestone projection (PL VI. Fig. 12, I). The 

 young Starfish has no other anus than that of the larva, which is placed on the verj 

 edge of the disk ; but, with the rapidly increasing deposit of limestone cells, it is soon 

 hidden from view, and I have not been fortunate enough to find it again in more 

 advanced young. I am therefore unable to say how the anus opens outside, though 

 it undoubtedly leads, at this time, through one of the many limestone cells. Owing 

 to the difficulty of tracing its opening in the dsedalus of round cells, I am not able 

 to state this positively, never having seen, from any point, discharges of fecal matters. 

 Like the madreporic body, it is not yet upon the abactinal area, but on the actinal 



