Fertilization and Development of Chaetognaths
Chaetognaths commonly called " arrowworms " or " glass worms " are translucid, carnivorous animals shaped like torpedos. They are very abundant in seas and oceans, only outnumbered by copepods, and represent an important food source for fish. Most of the 150 species of chaetognaths are planctonic (Sagitta). Their phylogenetic position is considered to be between protostomes and deuterostomes.The Chaetognath life cycle is about 1-3 months and a few benthic species (Spadella) can be raised easily in the laboratory. Chaetognaths are hermaphrodites but mate between individuals; their reproduction involves a spectacular exchange of packets of sperm. They are particularly well suited for studying gametogenesis and the germ line, as had been noted by Wilson in 1905 in " Cell Development and Inheritance ". They possess a particulate germ granule that is inherited in one of the first two blastomeres and by the first four germ cell precursors. The germ granule cycle and formation can be directly observed in Spadella and Sagitta, species which contain a Vasa-like protein (D.Carré, C.Djediat, C.Sardet (2002) Formation of a large vasa-positive Germ Granule and its inheritance by Germ Cells in the enigmatic Chaetognaths. Development, 129, 661-670).
Germ granule and germ cells in Chaetognaths
  Internal fertilization in Chaetognaths 
  Mating by exchanging sperm packets
  Cross fertilization and feeding behavior