PISCES 



143 



the species are deep-sea forms. One genus, Fierasfer (Fig. 80), is 

 remarkable on account of its commensal life as a lodger in the vari- 

 ous cavities of echinoderms and mollusks. One species, according to 

 Boulenger, enters its host, a holothurian, through the anal opening, 

 and lies with the head only protruding. From time to time it dashes 

 out after its prey and returns to its shelter to eat it. This could 



FIG. 80. Fierasfer acvs, penetrating the anal openings of holothurians, 2/i 

 natural size. (From Boulenger, after Emery.) 



hardly be interpreted as a case of symbiosis, for there can be no 

 mutuality in the arrangement. 



Stickle-backs, Pipe-fishes, and Sea-horses (Catosteomi) .' This 

 well-defined group of peculiar fishes exhibits a wide range of spe- 

 cialization and senescence. The Stickle-backs themselves (Fig. 81), 

 apart from their side-armor and prominent spines, are quite general- 

 ized in their proportions. Nothing less fish-like, however, could well 

 be imagined than some of the extreme Sea-horses, which look more 

 like gargoyles than real animals. The typical Sea-horse (Fig. 82) 

 might be compared with a knight of a set of chessmen, with a long, 

 coiled tail instead of a base. The Pipe-fishes may be considered as the 



