Round 2 - Chordata - Leptocardii
(Sources - 1, 2, 3, 4)
Our first chordates belong to the class Leptocardii, commonly called “Lancelets.” This is a small class consisting of one family, Branchiostomatidae, and 32 known species.
Lancelets are “fish-like”, filter-feeding, invertebrate chordates. Their body is translucent, without any limbs, and one poorly developed tail fin. They are not strong swimmers (see gif below). They have “gill-slits” that are for feeding only, not respiration. Their mouths consist of oral cirri: tentacle-like strands that act as sensory devices and filter water as it passes through the body. Most of their time is spent half-buried in the sand of the ocean floor, with their front end protruding, filtering plankton into their mouths and gill-slits. They have multiple light receptors which include Joseph cells, Hesse organs, a single anterior eye, and a lamellar body. Lancelets biofluoresce green inside their oral tentacles and near their eye spot. Some species also glow from the tail and gonads. These green fluorescent proteins may play a role in attracting plankton towards their mouth. Lancelets have a notochord but no skeleton, other than some cartilage stiffening their gill slits, mouth, and tail. They are relatively small, around 2.5 to 8 cm (1.0–3.1 inches) long.
All lancelet species seem to have males and females, though rare instances of hermaphroditism have been reported in two species. One species, Branchiostoma belcheri, was documented transforming from female to male. They reproduce via external fertilization, and only during their spawning season, which varies between species. Shortly after sunset, spawning lancelets will release eggs and sperm into the water column. Upon hatching, larvae are asymmetrical, with a mouth and anus on one side of their body and gill slits on the other. After metamorphosis, they become more symmetrical, though there are still some asymmetrical internal organs.
Lancelets may provide insight into the origin of vertebrates. They contain many similar organs to modern fish, albeit in primitive forms. Fossil chordates, such as Pikaia and Cathaymyrus from the Cambrian, and Palaeobranchiostoma from the Permian, have been suggested to be closely related to lancelets. However, modern lancelets probably did not appear until the Cretaceous or even Cenozoic and, despite their primitive form, are likely more derived than we first thought.
(bonk)
How do you feel about Leptocardii? (Remember to vote for your favorite animal within this group, not how you feel about the group as a whole!)
One or more of my favorite animals is in Leptocardii
I love one or more of these animals
I like one or more of these animals
I am neutral about all of these animals
I dislike all of these animals
I hate all of these animals
Propaganda under the cut:
