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TEGUMENTARY ORGANS 



irregular bands of elastic tissue, in which the remains of the primitive 

 endoplasts may be seen (as in fibro-cartilage), whose strongest fibres- 

 are horizontal, though they send out others irregularly in all directions. 

 The perpendicular columns are likewise composed of bundles of pale- 

 elastic fibres {^fig. 319. B), and if the intersection of the horizontal 

 with the vertical divisions be carefully examined, it is seen that the 

 former are, as it were, given off by the latter, which thus gradually 

 break up and thin out, terminating above and below in the elastic 

 fibres of the unstratified superficial and deep layers. A horizontal 

 section of this portion of the enderon presents a very peculiar appear- 

 ance, the transparent vertical columns 

 looking like radiating spaces, as 

 which they were, in fact, at first 

 described. 



Pigment of the enderon. — The en- 

 deron presents scattered masses of 

 pigment, sometimes contained in cells 

 and sometimes free, in many Inverte- 

 brata (Annelids, Trematoda, Echino- 

 derms, Crustacea, Mollusca). In other 

 Invertebrata and in the higher Ver- 

 tebrata, the pigment is confined to 

 the ecderon. In Fishes and Reptiles, 

 however, a well-marked layer of 

 pigment lies at the surface of the 

 enderon in the form of scattered 

 granules and of irregular more or less stellate masses which are not 

 enclosed in cells. The silvery lustre of the skin of fishes is due to- 

 minute rods which constitute a layer at this surface, and should 

 probably be regarded as a peculiar form of pigment granules. 



In the Cephalopoda and some Gasteropoda among the Inverte- 

 brata, the integument undergoes during life the most extraordinary 

 variations of colour, becoming overspread with successive clouds of 

 the most vivid hues. These are produced by the contraction and 

 expansion of peculiar sacs — the chromatophora — containing masses of 

 pigment granules. According to H. Miiller, (whose observations I have 

 recently had the opportunity of repeating,) these are sacs attached to- 

 whose walls are contractile fibre cells arranged radially, and frequently 

 anastomosing with those of other cells. They do not always contain 

 pigment, but frequently present a distinct nucleus. Several layers of 

 these chromatophora of different colours are frequently disposed, one 

 over the other, in a given portion of the skin, and produce by their 



Fig. 319. — Enderon of the Skate. 



