CHAPTER VII 



THE SKIN AND SCALES 



The skin of the Cyclostomata and Fishes consists (1) of the 

 epidermis, formed of several layers of epidermic cells, which are 

 constantly being recruited by the division of the cells of the basal 

 layer ; and (2) of a stratum of connective tissue with intermingled 

 unstriped muscle - fibres, blood-vessels and nerves, which con- 

 stitutes the deeper layer or dermis. From the epidermis are 

 formed the various unicellular or multicellular glands with which 

 the skin is provided ; and from one or both of the skin layers 

 originate the different calcareous structures which constitute the 

 hard exoskeleton. 



In the Cyclostomata the epidermis is particularly rich in 

 goblet-shaped, mucus-secreting, gland-cells. The Myxinoids also 

 possess numerous pockets of so-called " thread-cells." In each of 

 these cells the protoplasm secretes a long spiraUy-coiled thread, 

 and under the influence of appropriate stimuli the thread is shot 

 out and unwound to a great length. The threads and the mucus 

 are so abundant that one of these animals will convert a bucket 

 of water into a thick mass of jelly. N"o scales or other hard 

 exoskeletal structures are present in any of the Cyclostomata. 



In Fishes mucus-glands are also abundant in the epidermis, 

 and to their activity is due the slimy mucus which lubricates the 

 surface of the body. They are specially numerous in the Dipnoi 

 (e.g. Protopterus), where, in addition, there are many simple multi- 

 cellular glands which secrete the " cocoon " or capsule in which 

 the Fish is enclosed during the dry season. From the epidermis 

 are derived the poison-glands of some Teleosts, and also the 

 "glandula pterygopodia " in relation with the claspers of the 

 male Elasmobranchs. The glandular structures in connexion with 



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