The role of gelatinous exoskeleton, horizontal septum, muscles and tendons

Abstract

Dorsal fin muscles vary inorientation; many have origins on the capsule above the skull and run near-horizontally and some bipennatemuscles have origins on both capsule and septum. Such bipennate muscle arrangements have not beendescribed previously in fishes. Fin muscles have hinged tendons that pass through capsular channels and radialcartilages to insertions on fin rays. The capsule is gelatinous (89.8% water) with a collagen and elastinmeshwork. Greasy in texture, calculations indicate capsular buoyancy is partly provided by lipid. Capsule,septum and tendons provide elastic structures likely to enhance muscle action and support fast cruising. The skin in thedifferent newborn marsupials and the monotreme hatchling had a similar structure (no hair follicles and nosebaceous or perspiratory glands) and was in all cases less developed than the skin of altricial eutherians. Thethickness of the entire skin (36–186lm) and its different layers, epidermis (6–29lm) and dermis (29–171lm) varied among the marsupial species and reflected the differences in size and developmental degree of theneonates. In the skin of all marsupial neonates and the monotreme hatchling, numerous superficial cutaneouscapillaries were encountered, some closely associated with the epidermis, indicating the possibility that the skinparticipated in gaseous exchange.