It is estimated that the length of the larval life, includ- 

 ing the period -vuhen transformation is occurring, varies from 

 iil to J4.7 months; or, for practical considerations, is four years. 

 This is predicated on the pealc of hatching occurring during the 

 the month of June and the fact that ncvrly-transformed sea lam- 

 preys leave the mud-banks to migrate do^mstream fron November to 

 April of the follovrilng year. Since the greatest doivnstream migra- 

 tion usually occurs in the month of March, it would seem that the 

 r.iajority 01 individuals are about U6 months old at the time of 

 migration. 



The possibility has not been considered in the preceding 

 estimates that after attaining full grovjth, the larvae may enter 

 a rest period for one year before transformation begins. Gage 

 (l?23) believed that larvae of the lake lamprey passed through 

 this 12-month rest period. He based this belief on the behavior 

 (time of transformation) of some aquarium held specimens and the 

 fact that he frequently found larvae "as long or longer than 

 many transformdng ones anj"- month in the year". 



Although I have never found a larva as large as the largest, 

 newly-transformed sea lamprej'", many larvae in my series are as 

 large as many of the transformed individuals. The distribution 

 of sizes in mj^ T;!ay series of aimocoetes indicated the presence 

 of many individuals still in the river in the spring which were 

 as large as the buHc of newly-transformed individuals leaving 

 the mudbanl:s during the preceding vrinter months. This would tend 

 to substantiate Gage's contention. I have no ejqDerimental evi- 

 dence for this species in Mchigan waters which vrould indicate 

 whether this rest period does occur. Until such time as this 

 evidence is obtained, the possibility must be considered that a 

 12— :'.ionth rest period should be added to my estimate of the length 

 of the larval life. 



Transformation of larvae into parasitic adults 



A detailed description of metamorphosis in Petromyzon marinus 

 has been presented by Gage (1923). Very profound anatomical 

 changes talce place -miiich adapt the adult for its f ree-swijnaing, 

 parasitic existence. The most obvious external feattires of this 

 change are as follows: (1) the U-shaped, hooded projections around 

 the mouth of the larvae fuse ventrally and develop into the cir- 

 cular sucking-disc of the adultj -the sieve apparatus of the lar\-ae 

 disappears] replacing it in the throat is a file-like tongue armed 

 •vvith horr^ teeth; numerous horny teeth appear also upon the circu- 

 lar sucking-disc (see frontispiece); (2) the deeply imbedded, rudi- 

 mentary eye appears at the surface and develops into a highly fxmC" 

 tional organ -which is one of the most distinctive features of the 

 newly- transformed adult; (3) bod3'- color changes from the various 

 shades of broti.'n of the larvae to a light blue-gray dorsally and a 



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