Ascidiacea 569 



percentage at least will continue to develop under the following condi- 

 tions (Berrill, 1935) . A glass T-piece is attached to the stem of a thistle 

 funnel, the whole is immersed in seawater and air bubbled slowly through 

 the T-piece so that a slow current of water passes into the mouth of the 

 funnel. This is arranged so that the mouth opens downward. A piece of 

 coarse bolting silk on which the embryos rest is attached to the mouth of 

 the funnel. The embryos are thus kept mildly agitated in a gentle 

 stream of water. For complete success the carbon dioxide tension of the 

 water needs to be increased slightly (Child, 1927). Whenever possible 

 it is better in the case of viviparous species to start cultures with the 

 tadpole stage. It is not essential that the active stage be used, for in 

 most forms immature tadpoles showing but a trace of sensory pigment 

 will continue development outside the parent. 



REARING POST-LARVAL AND YOUNG ADULT STAGES 



In order to rear later stages of ascidians it is essential that the tadpole 

 larvae become properly attached to some surface. Tadpoles that fail to 

 become attached, while they may metamorphose perfectly, will not con- 

 tinue development and growth. Most swimming tadpoles if kept in a 

 large vessel of seawater, such as an inverted bell jar, will become at- 

 tached. It is often an advantage to lay flat pieces of glass in the vessel 

 so that the forms which become attached to them may be studied with- 

 out damage. 



If tadpoles have been reared in small vessels they should be trans- 

 ferred while still actively swimming to the larger vessel in which they 

 are expected to become attached and grow. An alternative method to 

 the use of large culture vessels is to submerge glass plates to which larvae 

 are attached in the sea in such a position that water movement will not 

 dislodge the larvae and detritus will not accumulate on the plates (Sim- 

 kins, 1924). 



In large vessels the water should not be changed once the larvae show 

 open siphons and functional stigmata. Replacement with water that 

 presumably is in better condition is rarely tolerated, so that it is im- 

 portant that the water used in the first place should be of excellent 

 quality. 



Only after attachment, and when the siphons and stigmata are func- 

 tioning, do ascidian larvae need food from external sources. Since mixed 

 assortment of small diatoms and algae etc. has been found to be at least 

 as satisfactory as a pure culture of Nitzschia, a coarse filtering of water 

 to remove the larger organisms is all that is necessary. A small quantity 

 of sodium nitrite and sodium phosphate should be added to the water 

 from time to time in order to ensure continued multiplication of the food 



