126 Dr. A. Krohn on the genus Doliolum and its species. 



it persists during almost the whole period of development of the 

 new creature, serves as an organ of locomotion, and begins to 

 wither away only when the young has reached its perfect deve- 

 lopment and independence*. 



The tail, however, dies away quite as Milne-Edwards has 

 already observed in the course of metamorphosis of Amouroucium 

 proliferum, and as I a short time since observed in larvse of Phal- 

 lusia mammillata obtained by artificial fecundation. 



The contractile central portion or axis of the tail, composed of 

 a simple series of rectangular, nucleated cells, is gradually re- 

 tracted from its sheath into the body of the young animal and 

 so becomes gradually shorter and shorter. Soon the young ani- 

 mal casts oiF its larval investment, and only slight traces of the 

 tail are left upon its ventral surface, close under the digestive 

 canal, in the form of a round body which soon disappears. 



The following observations will afford more detailed evidence 

 of the above view ; they were made upon separate, not yet fully 

 developed individuals of D. Nordmanni. 



To all these individuals the tail was still attached ; in some it 

 remained in all its integrity, while in others it had begun to dis- 

 appear. The whole, tail and animal, was surrounded by the lar- 

 val tegument, a very thick, glassy membrane, which must not be 

 confounded with the mantle, which is closely applied to the body 

 of the young animal. This could be readily distinguished from 

 the homogeneous larval tegument by the granules imbedded in 

 its substance. The larval tegument was about a line long, and 

 drawn out at each end into a tolerably acute point. The rela- 

 tively short and very thin tail, or rather its wasted axis, appeared 

 articulated from the presence of the above-mentioned cubical 

 cells, and external to these a thin muscular layer was perceptible, 

 whose fibres ran longitudinally from the root to the pointf. The 

 root projects far into a vesicular appendage attached close under 

 the intestinal canal, and filled with a clear fluid, which is pro- 

 bably only a dilatation of the second tunic [Leibes-schicht) , and 

 diminishing pari passu with the tail, collapses, and at length dis- 

 appears. The young animal appears in most specimens to be 

 already so far developed, that all the organs and the lobes of the 

 anterior aperture (which are at first turned inwards, and only 



* The animal described by Job. Miiller as Vexillaria fiabellum (Archiv, 

 1846), and considered by him to be probably the larva of Amouroucium 

 proliferum, is, according to my observations, an incompletely developed 

 Ascidian, whose tail, as in Doliolum, appears to persist until the perfect 

 form is nearly assumed. The perfect, as yet unknown animal will probably 

 be found to agree with Doliolum in its mode of life. 



t This layer of fibres seems to be wanting in no Ascidian larva. In the 

 tail of the Vexillnrite it has been already quite correctly described by 

 J. Miiller. It perfectly accounts for the rapid movements of the tail. 



