FISHES. 261 



])ostcrior aperture. Sometimes it penetrates through this head foremost, but gener- 

 ally enters in a characteristic manner. By its anal aperture the holothurian expels 

 and sucks in water. The fish, during the expulsion of the water, pushes its head into 

 the orifice, and curves its tail to one side, and then, by a rapid recoil movement, intro- 

 duces itself, tail foremost, into the intestinal canal, ])ushing farther and farther in with 

 every suction of its involuntary host. From the intestine it penetrates into the pul- 

 moiiarj' passages, and thence, after their rupture, into the perivisceral space. It re- 

 mains, however, near the anus, and protrudes its head, when hunger impels, in seareb 

 of food. It is therefore neither a true parasite, since it does not feed on its host, noi 

 a commensal, as it does not share the food of its host, but simply a lodger or tenant 

 at will. 



The last three families constitute a super-family — OPIIIDIOIDEA — well marked 

 off from all others by anatomical, especially osteological, characters. Others have been 

 associated with them, but very little is known al)<)ut them. Such are the Bkotuloph- 

 iDiu^, CoNGROGADiD^, and Ceedalaid^. 



Two other families ajjproximated to theOphidioidea are ATELEOPODiD.i: and Xeno- 

 cEPHALiD.E, but our information regarding them is in even a more unsatisfactory con- 

 dition than about the others. 



The su])er-family AMMODYTOIDEA, and family Ajuiodttid.e, are a group of 

 doubtful relations, although generally ])laced next to or among the Ophidioidea. They 

 have a long, sub-cylindrical, or slightly compressed body, graduating into the tail ; scales 



Flu. 14s. — Ammodi/tts amertcmius, sand-eel, sand launce. 



often arranged in oblique folds, the lateral line near the back, the head pointed, and with 

 the lower jaw extended forwards, and the operculum and sub-operculum much 

 enlarged ; the dorsal fin is composed solely of articulated rays, and occupies most of 

 the back; the anal fin is confined to the posterior half of the body; the caudal fin is 

 furcate; the pectoral fins are high on the scapular arch, and have all the rays branched, 

 and ventral fins are entirely absent. The species are rather few in number, and mostly 

 confined to the northern temperate seas, although a peculiar generic type (Bleekeria), 

 closely related to the cold-water forms, is found along the Indian shores, but very 

 sparingly. The species are known to the English-speaking people mostly under the 

 name of launce, lant, or sand-eel. 



Three S])eeies are found along the British shores, but one of them is very rare, 

 an inhabitant of deep water ; two species {Amrnodytes americcuius and A. dul>ii(i>) occur 

 along the American coast. The habits of all are essentially similar. They live on sandy 

 shores, congregate together in large numbers, and, when alarmed, take to the sand, in 

 which they rapidly plunge and force themselves, and remain concealed. On account of 

 their large numbers they form a very essential portion of the food of many fishes, and 

 especially the cod-fishes, and they are also much used as bait. They rarely reach a 

 length beyond a foot. They are chiefly captured at ebb-tide by digging into the sand, 

 and hoes and rakes, as well as sjiades, are put into requisition for this purjiose. 

 " Sand-eeling excursions by moonlight, at the low spring tides iu the sandy bays of 



