Palaeos:		Chordata
The Vertebrates		Chordata

Chordata

Urochordates

Chordata
|--Urochordata
|  |--Larvacea
|  |  |--Oikopleuridae
|  |  `-+--Kowalevskiidae
|  |    `--Fritillaridae
|  `--+--Ascidiacea
|     `--Thaliacea
`--+--Cephalochordata
   `--+--Haikouella 
      `--Vertebrata 

1. Chordata
2. Fritillaridae
3. Kowalevskiidae
4. Larvacea
5. Oikopleuridae
6. Urochordata

Descriptions

Chordata: Pikaia. 

Range: from the Tommotian, perhaps earlier

Phylogeny: *: Urochordata + (Cephalochordata + (Haikouella + Craniata)). 

Characters: body bilaterally symmetrical; $ single, dorsal, hollow nerve cord, usually with an enlarged anterior end (brain); nerve cord with pairs of nerves branching from it at more or less regular intervals and running to muscles; $ pharyngeal pouches, bars and/or slits (embryologically all the same thing); body (or post cranial body if a head is present) segmented, including segmented muscles; $ notochord, a cartilaginous rod running underneath, and supporting, the nerve cord and dorsal to gut; $ muscular tail projecting posterior to anus; three embryonic germ layers and well-developed coelom; ventral heart, with dorsal and ventral blood vessels and closed circulatory system.  

Image: Pikaia a possible chordate from the Early Cambrian Burgess Shale.

Note: many of these characters may not be present during some phase of the life cycle.

Links: Phylum Chordata; Introduction to the Chordata; Phylum Chordata Overview; Chordata; Chordata. The Columbia Encyclopedia, Sixth Edition. 2001; Chordata; biodidac; General Zoology - Phylum Chordata; Phylum Chordata; M17.htm; Chordate-classes; Encyclopedia.com: Chordata.  ATW080123.

Urochordata: (= Tunicata) Tunicates. Yarnemia?  (a pre-Cambrian form)

Range: from the Jurassic 

Phylogeny: Chordata: (Cephalochordata + (Haikouella + Craniata)) + *: Larvacea + (Ascidiacea + Thaliacea). 

Characters: Marine suspension feeders; ascidian embryo and larvacean adults have ciliated pharyngeal slits, notochord, muscular tail, dorsal nerve cord directly dorsal to notochord; dual nervous system with different nerves enervating segmented (somatic nerves) and unsegmented (visceral nerves) regions; segmented muscles (myomeres) enervated by dorsal nerve cord; locomotion by contraction of alternate sides as in vertebrates; exhibit some allorecognition (i.e. immune-type response). 

Links: Urochordates; Chordata; The Urochordata; Chordata : Subphylum Urochordata; OceanLink; Kennesaw.

References: Swalla et al. (2000). 

Image: © 1999 BIODIDAC and used by permission.  ATW080123.

 Larvacea: ( = Appendicularia): Planktonic (0.5 - 5 cm) forms with adult similar to ascidiacean larvae.

Range: Recent. 

Phylogeny: Urochordata: (Ascidiacea + Thaliacea) + *: Oikopleuridae + (Kowalevskiidae + Fritillaridae). 

Characters:  Individuals secrete acellular gelatinous "house" with incurrent, excurrent and caudal openings; incurrent duct has "screens" to keep out large particles and feeding screens or filters (again, acellular and extrasomatic); brushes food particles into mouth with cilia; endostyle secretes mucous to trap food particles; may have pharyngeal slits (respiration?); food passes through U-shaped digestive tract and out anus and excurrent duct; anus and tail are ventro-medial; tail long, with notochord; action of tail and cilia create current for filter feeding; dorsal nerve cord present throughout life; changes in "house" relatively frequent (as short as a few hours); generation time also 1 - 2 days; largely monoecious and protandrous (see the Glossary -- that's what its there for); able to feed on nannoplankton (on the order of 1 µ); non-cellular circulatory fluid and single heart present in some forms. 

Links: M20.htm; Blank (French); Nice papers on gelatinous zooplankton; gephrokapusa (image of Oikopleura; text in Japanese); Urochordates; Z250 (2000) - Deuterostome Clade; Earthlife.net; Larvacea.

The Earthlife site has a wonderful plain-language introduction to the Larvacea, with a good diagram.  Conversely, BIODIDAC has an excelent diagram (labelled in French), but no text.  A Spineless Column by Ronald L. Shimek, Ph.D. Tunicates or Sea ... has a very nice discussion of tunicates generally.  A number of sites touch briefly on the Larvacea in evolutionary context, e.g..: Home page and Chordata.  References to the literature are available at Pelagic Tunicate References.