CA TA GEA T ESfS. 



215 



to some foreign object, and then undergoes a remark- 

 able series of retrogressive changes, which convert it 

 into the adult ascidian. The tail atrophies, until noth- 

 ing is left but some fatty cells in the posterior part of 

 the trunk. The adhering papillae disappear and are 

 replaced functionally by a growth of the test over neigh- 

 boring objects. The nervous system with its sense- 

 organs atrophies, until it is reduced to the single small 

 ganglion placed on the dorsal edge of the pharynx, 

 and a slight nerve-cord running for a short distance 

 posteriorly. Slight changes in the shape of the body 

 and a further growth and differentiation of the branchial 

 sac, peribranchial cavity, and other organs now pro- 

 duce gradually the structure found in the adult ascid- 

 ian (Herdman). It is, however, to be noted that in 

 the order Larvacea, this retrograde metamorphosis 

 does not take place. It embraces the single family 

 Appendiculariidae, which includes Tunicata which pre- 

 serve the tail, notochord, and other larval features, and 

 lead a free-swimming existence in the ocean. 



On the Tunicata, Herdman makes the following 

 general observations. " (i) In the ascidian embryo 

 all the more important organs (e. g. notochord, neural 

 canal, archenteron) are formed in essentially the same 

 manner as they are in amphioxus and other Chordata. 

 (2) The free-swimming tailed larva possesses the es- 

 sential characters of the Chordata, inasmuch as it has 

 a longitudinal skeletal axis (the notochord), separat- 

 ing a dorsally placed nervous system (the neural canal) 

 from a ventral alimentary canal (archenteron) ; and 

 therefore during this period of its life history the ani- 

 mal belongs to the Chordata. (2) The Chordata larva 

 is more highly organized than the adult ascidian, and 

 therefore the changes by which the latter is produced 



