On the Life-History and Development of the Genus 

 Myzostoma (F. S. Leuckart). 



By 



John Beard, 



Berkeley Fellow of Owens College, Manchester. 



With Plates 31 and 32. 



Historical Introductioii. 



The genus Myzostoma has from the time of its first discovery been 

 one of the most doubtful genera among the Invertebrata. Of the ana- 

 tomy we have in past years learnt much , especially by the researches 

 of Semper 1 and Graff^. Biit the embryology, which as is well known 

 usually gives good criteria for determining the position of a group, was 

 almost unknown to previons observers. 



The first stage in the development recorded, was an advaneed one 

 observed by Se]\iper3. It had a simple, imbranched alimentary canal 

 and two pairs of legs. The discovery of this larva was of no importance 

 for determiniug the systematic position of the group. Indeed it taught 

 very little , and led rather to erroneous comparisons with the Nauplius 

 larva of Crustacea. In 1866 Metschnikoff ^ described accurately the 

 segmeutation of the q^^ of M. cirriferum , and figured a ciliated larva 

 of a simple form — little more than a planula. 



He further described some of the later stages in which the larva is 

 already in possession of its host. In both these cases the larva has per- 

 manent legs, in the one case two pairs, in the other the full complement 

 of fi ve. Both had a proboscis, and simple unbranched alimentary canal. 

 But although Metschnikoff's paper contained the description of a not 



* Semper , Zur Anatomie und Entwicklungsgeschichte der Gattung Myzo- 

 stoma (Leuckart). Zeitschr. f. wiss. Zool. Bd. IX. p. 48—64. Taf. III u. IV. 



2 Graff, Das Genus Myzostoma. Leipzig 1S77. 



3 1. c. p. 60. 



^ Metschnikoff , Zur Entwicklungsgeschichte von 3iyzostomum. Zeitschr. 

 f. wiss. Zool. Bd. IX. p. 236—243. Taf. XIII A. 



