Yaluscar

The Yaluscar (repochemoidea) or walking clam is a bizarre superfamily of bivalve which has evolved to have a body plan more resembling velvet worms or arthropods than clams. 

EVOLUTION

Yaluscar evolved from small, burrowing clams that adapted to be much like the Holocene's shipworms, burrowing in driftwood and stormrafts and evolving a much more worm-like body plan. Around 7 MYH (million years hence) Some of these worm-like bivalves abandoned their woody habitats and lived in the water column and on the seabed, and they eventually evolved legs to allow them to crawl on the seafloor more efficiently.

BIOLOGY

Yaluscar have unique biology for bivalves. Adults are around 3 inches long, and they have 10 legs and slowly crawl across the seafloor, feeding on detritus. They have an incurrent siphon, through which they breathe and consume food, and an excurrent siphon, through which they excrete water and waste. The excurrent siphon is positioned in a unique spot of their body, as it is on their back, rather than their rear end, where most bilaterally symmetrical animals have their equivalent of an excurrent siphon. Around their incurrent siphon are four pinhole eyes. Rather than using lenses, their eyes function like pinhole cameras, much like the eyes of the extinct nautilis. Unlike other bivalves, they have developed a centralized nervous system, bearing a ring-shaped brain at the front of their body near their eyes that surrounds the incurrent siphon, similar to the brains of squid. At the back of their body is a vestigial shell which hasn't quite gone away, at least not yet.

REPRODUCTION 

Yaluscar are hermaphrodites, and every adult individual produces both sperm and eggs. While they do sometimes self-fertilize when they can't find others to reproduce with, they typically reproduce sexually. They will typically only release one type of reproductive cell while mating with another yaluscar. 

They mate through broadcast spawning. Both individuals release their gametes at the same time and sperm cells will meet egg cells by chance. Mere hours after fertilization, the eggs will have become simple embryos, which go on to become juveniles which grow to adulthood in around 3 years.