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 
protozoa upon which the animal feeds. The Larvacea or Appendicu- 
larians 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, Otkopleura, 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 
apermanent 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 1. Ascidize Simplices. Solitary fixed forms which rarely 
bud; when colonial, each individual has a separate test. As- 
ctdta, Phallusia, Czona. 
Sub-order 2. Ascidiz Composite. Fixed Ascidians which repro- 
duce by gemmation, the individuals being embedded in a 
common investing mass. Sotryllus, Polyclinum. 
Sub-order 3. Ascidize Luciz. Free-swimming Ascidians which re-- 
produce by gemmation to form a colony, having the shape ot 
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.  Dololum, 
Anchinia. 
(4) 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 
