Oh the Life-History and Development of the Genus Myzostoma. 561 



Chance is an element, which enters into the life history of other 

 parasites hesides Myzostoma. One need hardly mention the eg-gs of 

 tapeworms etc. 



With its acqiiisition of a host the larva is led to further development, 

 the end of which is the attaiument of the adult form. The euticula 

 which is not so well developed in the earlier larvai stages attains now 

 to some thickness (fig. 45) . The ciliatiou is of the same character as 

 that of the adult. It is no longer confined to definite regions of the body 

 but as in the adult consists of cilia arranged over the whole body in 

 branches or gioups. 



Certainly the most striking changes consist in the development of 

 the legs. They are formed in the hind region of the larva. This region 

 without taking on at first external segmentai characters begins to form 

 segmental appendages. The legs are formed from before backwards, 

 in what are probably epiblastic sacs. At first the setae are straight and 

 without hooks, but they soon become hooked, the direction of the hook 

 being inwards towards the median line. The anterior pair are first 

 formed , and then the other four pairs in succession from before back- 

 wards. In addition to the external phenomena observed in the formation 

 of the limbs , there are certain internai processes going on which con- 

 cern the mesoblast. It was poiuted out when discussing the larva of 

 five days old that certain cells of the mesoblast remain in an embryonic 

 state, apparently without functìon in the larva (figs. 27, 28). 



These cells, which occupy a position on each side of the alimentary 

 canal in the hinder part of the larva, now become active, divide rapidly 

 and form a mass of cells, which are well seen in figs. 41, 44 and 48. 

 With the formation of the first pair of legs a certain portion of this mass 

 on each side becomes segmented ofiF from the rest to form the future 

 musculature of the legs (fig. 48) . So too in the case of each additional 

 pair of legs. Thus the mesoblast becomes segmented. A little later this 

 internai segmentation extends in part to the external portion of the 

 body, at least in M. cirriferum (fig. 48). But in later development the 

 external segmentation is not obvious and in this absence of external 

 segmentation Myzostoma resembles Polygordius. 



During the whole period of the development of the legs the alimen- 

 tary canal is quite simple in character, and the stomach has no diverti- 

 cula. The proboscis increases in size and becomes more retractile than in 

 the earlier stages. It is only sometime after the complete complement 

 of legs is formed that the alimentary canal acquires its diverticula. 



Mittheilungen a. d. Zoolog. Station zn Neapel. Bd. V. 37 



