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 spirally-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. No 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 “coccon” 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 
182 
