Fishes of the Western North Atlantic 21 



Additional Description. Rostrum, continuous with dorsal fin, varies in shape from 

 very narrow both above and below the notochord to more rounded in shape, and marked 

 off by definite notches both dorsally and ventrallyj dorsal fin-ray chambers from 170 to 

 iSoj preoral tentacles 21 to 29; intertentacular membrane much higher around the ven- 

 tral side of oral hood than laterally, where the tentacles on either side are interconnected 

 only near their bases/" Median fin (dorsal and ventral), posterior to atriopore, paddle- 

 shaped in some specimens (wider ventrally than dorsally), narrowing rather abruptly be- 

 tween anus and last few myotomes; however, it is narrower in some, with a more gradual 

 transition to the caudal process, there being a wide range of variation in this respect, even 

 among specimens of a single lot, as illustrated in Fig. 3, D, E; the distance from the anus 

 to tip of caudal process nearly twice as great as from last myotome to anus; myotomes 42 

 to 46 anterior to atriopore, 8 to 9 between atriopore and anus, 1 1 to 14 posterior to anus, 

 total number 62 to 68 ;" gonads 26 to 29, in a single series on the right-hand side. 



Color. This has not been described for living specimens. 



Size. Nineteen mm. is the greatest length yet recorded for Atlantic specimens.*" If, 

 however, the Lancelets recorded as A . lucayanum from the Philippines are actually iden- 

 tical with the western Atlantic form, then the species grows larger in the Far East waters, 

 for lengths up to 30 mm. have been reported there. 



Developmental Stages. In larvae of 6 mm., with only 22 pairs of gill openings, the 

 caudal extremity is expanded as a rounded fin; by the time the number of gill openings has 

 increased to 27 pairs it has become pointed, after which it elongates to the adult form." 



Habits. This species, like other Lancelets, lives much of the time buried in the sand. 

 But apparently it emerges more freely to swim about, for large numbers have been taken 

 in tow nets at or near the surface in Bahaman waters; they are taken most abundantly dur- 

 ing the early part of the ebb when the tide has been high about nine o'clock in the evening; 

 rarely are they taken in the daytime, or late at night. In aquaria they seldom leave the 

 sand in the daytime. Experiments have shown them to be negatively phototropic. The pos- 

 terior part of the body has considerable power of regeneration if cut off just posterior to the 



+0. In the original account of A. lucayanum, Andrews (Stud. Biol. Lab., Johns Hopk. Univ., Zool., 5, 1893 : pi. 13, 

 fig. 6) pictures the median ventral tentacle as considerably shorter than those next to it, with the membrane join- 

 ing it to them lower than that which joins the next three or four tentacles; Kirkaldy (Quart. J. micr. Sci., 37, 

 1895: 318, pi. 34, fig. 3), on the other hand, describes and pictures it as entirely frce'from the neighboring pair. 

 Forster-Cooper (in Gardiner, Fauna Geogr., Maldive Laccadive Archip., t, 1903: 348, fig. 76) shows the 

 membrane as notched where it connects with the ventro-median tentacle. But the membrane is higher there in a 

 specimen from that same region that we have examined; it is so described and pictured also by Franz (Jena Z. 

 Naturw., 5,?, 1922: 429, 430, fig. 321) for one from the Philippines. Evidently, then, the difference in this 

 respect is not geographic. Unfortunately, however, our West Indian series are not in good enough condition to 

 clarify this point. 



41. Parker (Bull. Mus. comp. Zool. Harv., ^6, 1904: 48) reported only four or five between atriopore and anus 

 for the Florida specimens which he named A. tnacricaudatum. But re-examination of these same specimens yielded 

 counts of eight to nine. 



42. Kirkaldy, Quart. J. micr. Sci., 57, 1895: 319. 



43. Larval development is described by Andrews (Stud. Biol. Lab., Johns Hopk. Univ., 5, T893: 219, pi. 13, 

 fig- 3. 5)- 



