62 Evolution and Distribution of Fishes 



II. Color, While some metanemerteans are gray, 

 silvery or brownish in color, like many fishes, they often tend 

 to assume gaudy hues of blue, green, pink, and crimson, 

 that seem to be protective in value (Burger p. 533) . Similar 

 tints and degrees of coloration characterize some fishes, 

 and in both groups the tints are due to pigment-cells em- 

 bedded in the skin, and which may change in tint with 

 change of environment. 



III. Skin. In metanemerteans this consists of an epi- 

 dermis that is richly ciliated; in Amphioxus the embryo 

 only is ciliated, while fishes, so far as known, have lost the 

 external cilia, but retain these over some internal surfaces. 



Glands, embedded in the epidermis, and resulting from 

 modification of Its cells, are very abundant amongst nemer- 

 teans. They may be single or arranged in packets, and 

 vary from elongate-tubular to ovate In shape. These main- 

 ly secrete the abundant mucus that surrounds the skin, but 

 may also exude a somewhat granular substance, and even 

 pigment materials. In Cyclostomata many goblet-shaped 

 cells excrete a very abundant mucus, that may even more 

 completely envelop each animal than In the last group. In 

 other fishes mucous excretions from epidermal gland-cells, 

 are usual, the amount of excretion reaching its climax In 

 the dipnoans, where as In some land nemerteans, a special 

 overflow may at times be changed Into a cocoon ( J2 : 201), 

 as shown by Parker for Protopterus. 



Thread cells or stinging cells occur in the epidermis 

 of some Nemerteans (e.g. Linens, Burger, p. 50), but 

 these are often restricted to the epidermal cells of the 

 proboscis as in Macriira and Cerehratuhis (Burger p. 212). 

 In cyclostomes the myxinoids are provided with urticating 

 cells that In structure and mode of action resemble those 

 of nemerteans. Some epidermal cells of young lampreys 

 secrete a digestive ferment, and such may represent modi- 

 fied urticating glands. Poison glands are found in not a 

 few fishes, but usually associated with surface spines 

 through which the poison Is excreted. The group of Weever 

 fishes (Trachinus) Is an example. 



