348 



structure as those described by W. M. Wheeler'' in Mtjxostoma gla- 

 brum Leuck., when retracted appearing as spherical or oval bodies with 

 walls so thick that the cavity is almost obliterated or reduced to an 

 irregular ramifying slit between the folds of the wall. 



The features described above seem to me of considerable interest 

 as they make a modification of the definition of the Myzostomidae 

 essential. 



The presence of six pairs of suckers in a species of Myxostoma ne- 

 cessitates a change in the generally adopted views on the morphology 

 of the Myzostomida. W. M. Wheeler in his important paper on the 

 sexual phases of these animals'* discusses the structure and homologies 

 of the suckers at some length. These organs (which he prefers to call 

 segmental sacs) he considers to be metamerie, lying laterally or dorsally 

 to their respective parapodia like the 'Seitenorgane' of Capitellidae and 

 for various reason he believes them to be homologues of these lateral 

 line organs or segmental sacs. In the course of his discussion of this 

 subject we find the following passage : — 



"The fact that in Myxostoma there are five pairs of parapodia but 

 only four pairs of segmental sacs, naturally leads to the question as to 

 what has become of the missing pair of sacs. The answer to this ques- 

 tion I believe we need not go far to seek: the third pair of the ori- 

 ginal five pairs of sacs has been converted into the so-called 

 penes. These are more or less prominent papillae lateral to and near 

 the bases of the third pair of parapodia. Each papilla is perforated 

 by a ductus ejaculatorius which widens proximally into a vesicula semi- 

 nalis. The latter receives the mature spermatozoa from the vasa de- 

 ferentia and these in turn from the ramifying testicular follicles. Bizarre 

 as the development of a male reproductive organ from a lateral line 

 organ may appear at first sight, I am nevertheless unable to see any 

 great difficulty in such a change of function. It is in fact easy to see 

 how the bottom of an eversible sac might acquire an opening into the 

 body cavity under the pressure of a great accumulation of spermatozoa; 

 the sac would then become reduced to a mere conduit'". 



The occurrence of a species of Myxostoma with six suckers renders 

 Wheeler's views on the subject ([uite untenable, for in this form we 

 find the number of jiairs of suckers or segmental sacs to be greater 

 than the number of pairs of parapodia, and we have to consider that we 

 are dealing with six segments instead of five. The problem before us 



3 W. M. Wheeler, 'The Sexual Phases oï Myxostomf/ . Mitt. Zool. Stat, zu 

 Neapel. Bd. XII. 1895-97. S. 281. 



4 W. M. Wheeler. T. c. p. 278. 



