CLASSIFICATION. 457 



exterior by two ventral ciliated slits, and there is no peribranchial 

 chamber. The nervous system extends into the tail region. A 

 relatively large cuticular "house" is formed as a secretion round the 

 animal ; it is periodically cast off and rapidly replaced. The house acts 

 as a most efficient filtering apparatus for capturing minute diatoms and 



Crotozoa upon which the animal feeds. The Larvacea or Appendicu- 

 irians are of special interest because they show little or no degeneration, 

 and retain throughout life the chordate characters which other Tunicates 

 lose during metamorphosis. Appendicularia, Oikopleura^ Fritillaria, 

 Megalocercus, Kowalevskia. 



Order 2. ASCIDIACEA 



Ascidians which may be fixed or free, simple or colonial, but which 

 in the adult have no tail and no trace of notochord. There is a large 

 branchial sac opening by many slits into the peribranchial chamber, 

 which communicates with the exterior by a single opening. There is 

 a permanent and well-developed cuticular test into which cells from 

 the body migrate. Many have the power of budding, and there is 

 frequently alternation of generations. 



Sub-order I. Ascidiae Simplices. Solitary fixed forms which rarely 

 bud ; when colonial, each individual has a separate test. As- 

 cidia, Phallusia, dona. 



Sub-order 2. Ascidiae Composite. Fixed Ascidians which repro- 

 duce by gemmation, the individuals being embedded in a 

 common investing mass. Botryllus, Polydinum. 

 Sub-order 3. Ascidiae Luciae. Free-swimming Ascidians which re- 

 produce by gemmation to form a colony, having the shape ol 

 a hollow cylinder, open at one end. There is one genus, 

 Pyrosoma^ widely represented, especially in tropical seas. 

 They are brilliantly phosphorescent, and some attain a length 

 of twelve feet. 



Order 3. THALIACEA 



Free-swimming pelagic forms, which may be either single or 

 " social," and in the adult are never provided with tail or notochord. 

 The muscles are in the form of distinct circular bands, which effect 

 locomotion by squirting out the water from the body. The test, 

 which may be well or ill developed, is always transparent. The life 

 history exhibits distinct alternation of generations, and there is some- 

 times polymorphism. 



(a) Cyclomyaria. Muscle bands form complete rings. Doliolum^ 

 Anchinia. 



(d) Hemimyaria. Muscle bands are in the form of incomplete rings. 

 Salpa, Octacnemus. 



RELATIONSHIPS 



The questions as to the origin of the Tunicates and the relations of 

 the orders are too difficult to be discussed here, but we may note that 



