Outlines of zoology / by J. Arthur Thomson.

  • Thomson, J. Arthur (John Arthur), 1861-1933.
Date:
1895
Catalogue details

Licence: Public Domain Mark

Credit: Outlines of zoology / by J. Arthur Thomson. Source: Wellcome Collection.

    715/864
    Order 2. Sirenia—Sea Cows. A small moribund order of sluggish, aquatic, vegetarian Mammals, in no direct way connected with Cetaceans, possibly related to Ungulates, but certainly primitive. There are two living genera, Halicore (Dugong), and Manatus (Manatee), and one recently exterminated (Rhytina). The Sirenia are sluggish animals, with massive heavy bones, a plump body, some oil, and sparse hair. They are aquatic, with fish-like form, no trace of hind limbs, flipper-like fore limbs, no external ear, valved nostrils, networks in the arteries (useful in prolonged immersion). They are herbi- vorous, and like others of similar habit have a chambered stomach, a long intestine, and a caecum. They are primitive, and with this fact may be associated the abdominal testes, the absence of distinct epiphyses on the vertebrae (as in Prototheria), and the small, rather smooth brain. The body is fish-like, the head rounded, the skin is thick and tough, with sparse bristles, mostly about the mouth. The paddle-shaped fore limbs have, at most, rudimentary nails ; there are no hind limbs. The skull is not like that of Cetaceans. The nasals are, at most, rudimentary. There are no canine teeth. There are chevron bones below the tail. There are no clavicles. The pelvis is rudimentary, and there is no sacrum. The brain is small and has few convolutions. The nostrils are valved, and lie at the end of the snout. The small eyes have imperfect eyelids, but have a nictitating mem- brane. There are no external ears. In the mouth there are horny crushing plates. The stomach is chambered, and there is a caecum. The ventricles are separated by a cleft. There are retia mirabilia on the arteries. The testes are abdominal in position. The uterus is bicornuate. Two teats lie behind the arm pits. The placenta of the dugong is zonary, wholly or in great part non-deciduate. The placenta of the manatee has not yet been investigated. The food consists of Algce and estuarine plants.