ORDERS OF TUNICATES 513 



development is in consequence much abbreviated, the tailed larva not 

 being represented. This embryo gives rise to a soUtary " nurse " form, 

 which by budding produces a chain of embryos. This chain is set free ; 

 its members become sexual, and, either while still united or after 

 separation, give rise to the eggs which develop into the nurse form. 



The remaining order of Tunicates includes minute simplified forms 

 like Appendicularia, also pelagic in habitat, but without any. power of 

 budding, and never forming colonies. These forms have a distinct tail, 

 which is bent at an angle to the body, and is the main organ of locomo- 

 tion. The mouth is at the anterior end ; the anus, which is distinct 

 from the atrial openings, is at the root of the tail. These atrial openings 

 lie slightly behind the anus, and are merely small ectodermic invagina- 

 tions communicating with the two gill-slits of the pharynx. They 

 correspond to the similar invaginations in the Ascidian larva. The test 

 may form a large investing " house," but it does not contain cells, and 

 is periodically cast and renewed. The important points as regards 

 internal structure are the presence of the notochord throughout hfe, and 

 the structure of the nervous system. The latter consists of a lobed 

 ganglionic mass above the mouth, and a dorsal nerve-cord extending 

 backward from this into the tail, where it is furnished with other 

 gangha. In connection with the cerebral ganglion there is a pigment 

 spot, an otocyst (auditory?), and a tubular process communicating with 

 the pharynx, and corresponding to the sub-neural gland and the cihated 

 duct of other Tunicates. We have already noted the simple structure 

 of the pharynx, which has but two gill-slits communicating directly with 

 the exterior. The same simphcity of structure is observable in the 

 heart, which is without any associated vessels. The hermaphrodite 

 reproductive organs he posteriorly, and open to the exterior by a 

 very fine duct on the dorsal surface. As contrasted with Salpa and 

 DoHolum, the animals are protandrous, and not protogynous. The 

 development is unknown. 



Classification.— 



Order i. Larvacea 



Free-swimming, pelagic, and solitary forms provided with a large 

 locomotor tail containing a notochord. The pharynx opens to the 

 exterior by two ventral cihated 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 capfuring 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, 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 



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