Sagittiferidae is a family of acoelomorph

Acoelomorpha

The Acoelomorpha are a disputed phylum of animals with planula-like features that were considered to belong to the phylum Platyhelminthes. In 2004 molecular studies demonstrated that they are a separate phylum, although their position in the tree of life is contentious; most researchers believe...

 worms.

Systematics

The family is divided into five genera (see taxobox) and contains about 20 species (Turbellarian taxonomic database, 2006). The affinities of the genera are as yet unclear. Some biologists prefer to exclude the genus Praesagittifera, leaving into the family only the genera with sagittocysts (Achaz et al., 2005).

Characteristics

Sagittiferidae are small flat worm-like animals. Several of them grow symbiotic algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

 Tetraselmis

Tetraselmis

Tetraselmis is a genus of phytoplankton. Tetraselmis has a very high lipid level; their amino acids stimulate feeding in marine organisms . Tetraselmis is green, motile, and usually grows 10 µm long x 14 µm wide.The species T...

in their body cavity and tissues.

Special characteristics include a ciliated or non-ciliated sac-like male antrum

Antrum

In biology, antrum is a general term for a cavity or chamber which may have specific meaning in reference to certain organs or sites in the body.In vertebrates, it may mean for example:...

. Most members have sagittocysts (Kostenko&Mamkaev,1990) on their skin. Sagittocysts are needle like bodies 18-50 microns long that contain some secretory product. They can be extruded from the body by an associated muscle. They are believed to play a role in defence and prey capture. Ultrastructurally, they have a vesicular core surrounded by a mesh of actin

Actin

Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...

 filaments.

Antrosagittifera

Contains the following species:

• Antrosagittifera corallina Hooge & Tyler, 2001

Convolutriloba

Contains the following species:

• Convolutriloba hastifera Winsor 1990
• Convolutriloba longifissura Bartolomaeus & Balzer, 1997
• Convolutriloba macropyga Shannon & Achatz, 2007
• Convolutriloba retrogemma Hendelberg & Akesson 1988

Sagittifera

Contains the following species:

• Sagittifera sagittifera (Ivanov, 1952)

Symsagittifera

Contains the following species:

• Symsagittifera bifoveolata (Mamkaev, 1971)
• Symsagittifera corsicae Gschwentner, Baric & Rieger, 2002
• Symsagittifera japonica (Kato, 1951)
• Symsagittifera macnaei (Marcus, 1957)
• Symsagittifera nitidae (Yamasu, 1982)
• Symsagittifera poenicea Kozloff 1998
• Symsagittifera psammophila (Beklemischev, 1957)
• Symsagittifera roscoffensis
  Symsagittifera roscoffensis
  Symsagittifera roscoffensis, formerly called Convoluta roscoffensis, is a free-living acoelomorph worm.-Appearance:Symsagittifera roscoffensis is a small flat worm. Due to the algae, Tetraselmis convolutae, which it assimilates into its body cavity, it has green colour...
  
  (Graff, 1891)
• Symsagittifera schultzei (Schmidt, 1852)
• Symsagittifera smaragdina Achatz, Gschwentner & Rieger, 2005