16 AKT. 11. — A. IZUKA : OBSERVATIONS OX THE 



The Change of the Immature {Atoca) into the Mature Phase 

 {Epitoca) — This begins to take place in the early part of September. 

 All individuals that have passed the summer do not, however, pass 

 nearly simultaneously into the epitoca. At the period of the 

 year just mentioned, the worms still vary in size and it is especially 

 the larger ones, as might be expected, which first begin to 

 manifest symptoms of the change. These are however by no 

 means of a uniform size. They are undoubtedly those which are 

 destined to swarm out under the first favorable circumstances 

 during the following; month. Others which swarm later in the 

 season undergo the change at a correspondingly retarded period. 

 Probably some of the smallest found in the season do not become 

 at all ready for swarming until the autumn of the following year; 

 for throughout the winter after the last swarming has taken place 

 in November, there are still to be found small immature worms, 

 though in an incomparably smaller number than in the spring 

 and summer. 



The successive stages of the change into the epitocous phase 

 are illustrated in PL I., figs. 1-5. To start with, fig. 1 shows 

 a well-grown representative atoca, before the change has set in. 

 In such specimens the sexes can not be determined by external 

 observation. This becomes however possible as the worm grows 

 markedly stouter, indicating the beginning of the change. The 

 sexes then begin to present a difference in color. The worms in 

 fig. 2, — in which figure, as also in several following figures, the 

 letters A and B stand respectively for the male and the female, 

 — the male (2 A) will be observed to be more dusky or more 

 brownish than the female (2 B), which is for the most part of 

 a bright red color, while both have grown considerably in breadth, 

 especially in the anterior region of the body as compared with 



