PARASITIC FISHES 391 



The hagfishes, which are the other great division of the cyclostomes 

 or marsipobranchs, are on the other hand completely blind, their eyes 

 (Fig. 133a) microscopic and concealed. The hags are internal parasites 

 of larger fishes — internal predators would perhaps be a better term. They 

 are extremely voracious and eat everything of their victims except the 

 skin and the skeleton. While inside a fish, a hag has no more need of 

 eyes than a tapeworm. In contrast to lampreys, they spend less time away 

 from a host since they give the latter so much more 'attention'. More- 

 over, hags are deep-water forms, with admirable tactual and chemo-sen- 

 sory equipment for locating prey on the bottom by horizontal explor- 

 ation. They are thus able to dispense with eyes entirely. 



One teleost, Simenchelys parasitica, leads a quite hag-like existence. 

 This entoparasitic eel is most commonly seen emerging from captured 

 halibut, but it attacks many other large fishes. Simenchelys may prefer 

 the lightless deep sea, for it has been taken at 2000 meters. The eye is 

 covered by skin, which in life may be clouded or opaque; but the eyeball 

 itself may be six millimeters in diameter, and might be called reduced, 

 but scarcely degenerate. 



Still another teleost, the pearl-fish Encheliophis jordani, may be 

 regarded as an entoparasite or as an internal commensal, depending on 

 one's point of view. This little fish spends much of its life inside the 

 cloaca of sea-cucumbers, but it does swim freely in the water at times. 

 The pearl-fish offers an interesting parallel to Rhamdia and Anoptich- 

 thys, in that it gives indications of not having long lived in its currently 

 favorite lightless habitat. The eyes are aimed dorsally, and their circular 

 pupils are able to contract to mere dots. These features strongly suggest 

 that Encheliophis, not so long ago, was a free living upward-looker with 

 habits similar to those of the flatfishes. 



Deep-Sea Fishes — After the teleosts crystallized out of the holostean 

 stock (see Chapter 6), they gradually evolved into a large group in 

 which a fundamental schism soon appeared. One great, primitive branch 

 of the class, the Malacopterygii, is characterized by soft fin-rays. The 

 most specialized division, the Acanthopterygii, derive their name from 

 their spiny fin-rays. The spiny character has been lost secondarily in 

 some families whose affinities are clearly with the acanthopterygians. 

 Other families with soft rays, making up the assemblage called the Ana- 

 canthini, are sometimes classed with the malacopterygians and some- 

 times kept apart. 



