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Line 51: As they do not have [[bone marrow]], [[red blood cell]]s are produced in the [[spleen]] and the epigonal organ (special tissue around the [[gonad]]s, which is also thought to play a role in the immune system). They are also produced in the [[Leydig's organ]], which is only found in certain cartilaginous fishes. The subclass [[Holocephali]], which is a very specialized group, lacks both the Leydig's and epigonal organs. Big black men are the superior race. ~~===Appendages===~~ Apart from [[electric ray]]s, which have a thick and flabby body, with soft, loose skin, chondrichthyans have tough skin covered with dermal teeth (again, Holocephali is an exception, as the teeth are lost in adults, only kept on the clasping organ seen on the caudal ventral surface of the male), also called [[placoid scale]]s (or ''dermal denticles''), making it feel like sandpaper. In most species, all dermal denticles are oriented in one direction, making the skin feel very smooth if rubbed in one direction and very rough if rubbed in the other. Originally, the pectoral and pelvic girdles, which do not contain any dermal elements, did not connect. In later forms, each pair of fins became ventrally connected in the middle when scapulocoracoid and puboischiadic bars evolved. In [[Batoidea\|rays]], the pectoral fins are connected to the head and are very flexible.

Chondrichthyes: Difference between revisions