Lagnodactylia
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Lagnodactylia (meaning "sexy finger") is a class within Asterzalia, which are distinguished by their mouths, digits, bilateral symmetry, and sexual reproduction. Many have reduced or completely lost the ancestral ability to filter-feed, as the presence of the mouth allows them to consume much larger prey much faster. They all possess some type of subdermal musculature apart from their ancestral intestine, which has developed into an abdominal wall in some lineages.
Terminology
Many traits of lagnodactyls have specific names which may be mentioned in their descriptions.
- Anal Arm: The "hind" arm, which is tail-like in some lineages. So named because the "rectum" runs through it, it also contains the reproductive organs, making the "anus" more of a cloaca.
- Anal Fingers: The digits of the anal arm. These are used to hold on during copulation.
- Raptorial Arm: The "front" arm or third leg, so named for its use to grasp prey in many Ceratoasterformes.
Biology
Feeding
Lagnodactyls ancestrally retain the ancestral "filter baits" of Asterzoa, though some branches have since lost these. In species that do have filter baits, they essentially function the same way as those in other asterzoa--they detect microscopic cells bumping into them, causing a pseudopod to shoot out of the bait's cell to absorb them. The ancestral state of lagnodactyls is to digest their food entirely within their cells, even food absorbed through the mouth.
Primitively, the mouth externally appears to be skin rather than something for feeding. The way it works is that, upon food being detected, it suddenly and rapidly turns inside-out, creating suction that pulls the meal inside. This essentially converts the mouth into a stomach until the food is absorbed or digested, at which point it pops back out, releasing any indigestible material that was caught. Many derived lineages, such as Astergnathidae and Asterostridae, have evolved jaws or other alternative methods of eating.
Reproduction and Development
Most Lagnodactyls mate by linking their anal arms together and joining "fingers" so that the male can transfer sperm to the female. Basically, they mate by holding hands. Ancestrally, they gave live birth, but some, such as the Grazestar and its descendants, have developed the ability to lay eggs.
All lagnodactyls have radially symmetric larvae or fetuses that undergo metamorphosis into bilateral adult forms. This transformation starts with the proportional lengths of the arms and any other parts that came in a set of 4 shifting towards their adult lengths and the first appearance of simple bilateral traits not present in newborn larvae. Afterwards, the shape of the arms and other parts that came in a set of 4 morph into their adult shapes. The development of features beyond those that were radial in larvae can vary between orders and even individual species; for example, the lateral mandibles present in some members of Ceratoasterformes do not appear until halfway through metamorphosis while their eyes are among the first bilateral traits to start developing.
Evolutionary Constraints
Lagnodactyls are unable to remove the radial stage of development. However, some have evolved to delay birth or hatching until metamorphosis is complete, as seen in Crestgills. Since the ancestral state is for their offspring to swim right out the moment they are capable, reproductive advancement is highly variable within the group and can take several evolutions to improve.
Subgroups
Order Asterovenatoformes
Primitive lagnodactyls which lack complex eyes and retain the external appearance of being radially symmetric, despite still being bilateral internally.
Order Ceratoasterformes
Jawless lagnodactyls with advanced coeloms and spikes or serrations in their mouths which allow them to chew their food.
Order Astergnathiformes
Jawed Lagnodactyls with advanced coeloms and internal vertebrate skeletons.
Order Asterbranchia
Lagnodactyls with three of their four arms modified into rather large and showy gills.