26 ART. 11. A. IZUKA : OBSERVATIONS ON THE 



are indistinct. Elongation of the hypodermis cells and a con- 

 sequent thickening of the layer they compose also occur close to 

 an artificial cut by which the hind part of a worm has been 

 removed, but in this case, the cells of the intestinal wall do not 

 show any sign of atrophy. 



The circular muscle of the dermal musculature is strongly 

 developed in the last segment, especially in its posterior part. 

 The longitudinal muscles end abruptly in this segment ; so like- 

 wise the ventral nerve cord, its fibres showing some signs of 

 defeneration. There is found no ring vessel connecting the ventral 

 longitudinal vessel with the dorsal, which rino-vessel is one of 



O 7 O 



the points characteristic of the anal segment in atocous worms ; 

 both the dorsal and the ventral blood vessels terminate freely 

 and apparently blindly. From what has been said it clearly 

 follows that the hindmost segment of the epitocous worm is not 

 the anal segment of the atocous. On the contrary there can be 

 no doubt whatever of its being an ordinary body segment, which 

 has undergone some change, as the result of the shedding away 

 of the more posterior segments. 



Some Other Points of Anatomical Differences between the 

 Atoca and the Epitoca. — The integument becomes thinner as the 

 worm passes into the epitocous phase. The cuticula measures 1- 

 lh (j- in thickness in the epitoca, while in the atoca it is 2-3/' 

 hick. The hypodermis is 10-13/-« thick in the epitoca, but 20- 

 25 !>■ in the atoca. The thinning is evidently due to the expansion . 

 of the general body cavity, in consequence of the development of 

 reproductive elements. 



Apparently for the same reason, both the dorsal and the 

 ventral longitudinal muscles in the epitoca are greatly thinned 



