Short answers, pulled from the story.
Chondrichthyes is the class of jawed fish whose skeletons are composed primarily of cartilage rather than bone. They are contrasted with Osteichthyes, or bony fish, and are distinct from all other jawed vertebrates within the infraphylum Gnathostomata.
Chondrichthyes is divided into Elasmobranchii, which includes sharks, rays, skates, and sawfish, and Holocephali, which includes chimaeras, sometimes called ghost sharks. Elasmobranchii lack opercula, while Holocephali are a highly specialised group that lacks several organs found in other cartilaginous fish.
Red blood cells in chondrichthyans are produced in the spleen, the epigonal organ (tissue surrounding the gonads), and in certain species the Leydig's organ. The Holocephali subclass lacks both the Leydig's and epigonal organs.
The Ampullae of Lorenzini are networks of small jelly-filled pores that function as electroreceptors, allowing cartilaginous fish to sense electric fields in water. They assist with finding prey, navigation, and detecting temperature.
Capture-induced parturition is premature birth or abortion triggered when pregnant sharks and rays are caught by fishers. Studies have shown it occurs in at least 12 percent of live-bearing sharks and rays, affecting at least 88 species, and it is rarely accounted for in commercial fisheries management.
The oldest unequivocal fossils of acanthodian-grade cartilaginous fish are Qianodus and Fanjingshania, from the early Silurian (Aeronian stage) of Guizhou, China, dating to around 439 million years ago. These are also the oldest known remains of any jawed vertebrates.