142 WYVILLE THOMSON, ON SYNAPTA INH^RENS. 



its course. The vesicle gradually becomes a rosette of ten 

 or eleven csecal processes, and at length five of these 

 processes declare themselves as the rudiments of the first 

 five tentacles, five more indicate the five ambulacral vessels, 

 and the eleventh the Polian ampulla ; all of these caeca, with 

 the dorsal vessel, branch from a common vascular ring, 

 the circular canal of the ambulacral system. The Auricu- 

 laria now alters rapidly in form. The sinuous lateral fringes 

 disappear, the body losing its Inlateral symmetry, becomes 

 lengthened and cylindrical, and encircled by five transverse 

 ciliated bands ; the provisional mouth and oesophagus are 

 obliterated, and the ambulacral circular canal passes round 

 the anterior extremity of the stomach. The stomach, the 

 intestine, and anus of the larva, seem to be adopted by the 

 rudimentary Echinoderm. The calcareous ring bisecting the 

 dorsal canal becomes more complicated, assuming the 

 structure of the madreporic tubercle ; the portion of the tube 

 external to it appears, at all events in one species, to be obli- 

 terated ; and the internal portion remains as the sand-canal, 

 an appendage to the vascular ring. The anterior extremity 

 of the body still forms a closed dome over the sprouting 

 tentacles. In this, which Miiller calls the " pupa" stage, the 

 embryo Holothuria thus becomes gradually developed within 

 a closed, transparent, ciliated sarcode-cylinder. In the next 

 stage the anterior vaulted end of the cylinder opens, the 

 tentacles are protruded, the new mouth comes into operation, 

 the ciliated bands disappear, and the animal assumes its final 

 form. 



Professor Miiller's observations were made upon two forms 

 of Holothurian larvte, which he believed he could always 

 recognise by characters which they severally retained through 

 all their metamorphoses. 



In one, eleven large transparent vesicles appeared in the 

 young Auricularia imbedded in the substance of the sarcode, 

 and a peculiarly formed echinated calcareous spherule occu- 

 pied the posterior extremity of the body. It was in this 

 species that Professor Miiller was able to trace the later 

 metamorphoses with accuracy. In the other form, very 

 perfect calcareous wheels ornamented the frills and auricular 

 appendages. 



From the close resemblance which these wheels bore to the 

 wheels of Chirodota, Professor Miiller thought himself justi- 

 fied in referring the second Auricularia to that genus. Dr. 

 Krohn (" Ueber die Entwickeiung der Seesterue und Holo- 

 thurien," 'Miiller's Archiv,^ 1853), afterwards succeeded in 

 rearing some of the larvse with calcareous wheels, in a tank. 



