destroyed, and the pigment passes into the blood channel. 

 The formed larva is set free from the membrane by the tail 

 movement. The body of the larva CFigure 48, B) in the anterior 

 end is supplied by three similar processes on the sucker. The 

 larva is soon attached at the anterior end and undergoes 

 transformation, one of its marks being the disappearance 

 of the tail. Milne-Edwards saw only that the axial part of 

 the tail is set free from its covered sheath and extends into 

 the body of the larva, but he did not elucidate the subsequent 

 fate of this formation. "By my observations," Krohn noted, 



the setting free and the extension of the tail axis , 

 the deep immersion by the tail in the body of the larva 

 only precedes the processes of reduction which it soon 

 undergoes. Directly after the extension, the tail axis 

 remains undamaged at the posterior part of the body. 

 It is situated here convoluted into a spiral coil . . . . 

 With the beginning of the development of the young 

 ascidian, this coil first disintegrates into a large 

 number of strips situated close to each other, which 

 then are gradually destroyed; the number and size of 

 the strips decrease, but the insignificant remnant 

 does not disappear entirely. (Figure 48, C) (pp. 318 - 

 319) 



Krohn himself considered his observations on the develop- 

 ment of ascidians incomplete, and he acknowledged only the most 

 essential changes. He described in particular the formation of 

 the vessels of the tunica and the development of the respira- 

 tory cavity, or gill cleft, and behind it the rudiment of the 

 digestive canal in the form of a loop-shaped canal . Somewhat 

 later three openings on the spinal side of the body appear: the 

 most anterior, the inlet into the respiratory cavity and 

 digestive canal, and two posterior which later merge together 

 in a common excretory opening. Simultaneously, the nerve 

 ganglia develop in an elongated formation in the middle of the 

 back near both pigmentation spots. Near the nerve ganglia the 

 rudiment of the muscular strands form and the dorsal fissure 

 appears. The digestive canal is differentiated into three 

 parts: a canal which opens into the respiratory cavity, stomach, 

 and intestine. In the walls of the respiratory cavity there 

 develop near the stomach the first branchial clefts with cilia 

 at the edges, and at the ventral fissure the heart develops, 

 possessing the form of a short duct. The metamorphosis is 

 completed by the specialization of the gill-clefts and the 

 development of siphons . 



591 



