566 John Beard 



pod. As in them the segmentation is complete but unequal, a gastrula 

 arises by epibole , the mouth probably arises on the site of the blasto- 

 pore , a ciliated larva , which later acquires special bands of cilia and 

 provisionai setae, leaves the egg. The fimctional larvai nervous system 

 arises as a thickening of the praeoral lobe. In the adult the skin is 

 that of a Chaetopod and the permanent limbs are easily derivable from 

 Chaetopod setae. The alìmentary canal is such as one often meets witb 

 among Chaetopoda. 



The nervous system is reducible to a series of fused ventral ganglia. 

 Supra-oesophageal ganglion , and possibly also the nerve collar have 

 disappeared in consequence of loss of sense orgaus etc. due to parasitic 

 habits. As a consequence too, of parasitism, the sexual organs have 

 undergone great changes , and along with that the segmentai organs 

 have partly disappeared. 



The ovaries are best regarded as arising from the epithelial lin- 

 ing of the body cavityi, which they have by hypertrophy completely 

 obliterated. The oviduct opening into a cloaca may perhaps originally 

 have been formed from two segmentai organs , for the cloaca is an epi- 

 dermic invagination , and if it were obliterated the oviduct would open 

 on the median line. The two male ducts are much more easily refer- 

 able both from position and structure to segmentai organs, which stili 

 open into the modified remaius of the body cavity. In fine I agree with 

 the great Russian naturalist Metschnikoff, who says^: »Ich betrachte 

 dieses Thier [Myzostoma] als Repräsentanten einer besonderen Chaeto- 

 poden-Gruppe, die Chaetopoda ectoparasitica heißen können.« 



It is indeed perhaps impossible to place the genus near any par- 

 ticular family of Chaetopods. In its development it has resemblances 

 to that of Nereis, and perhaps may be classed as a special family of 

 the Errantia. And so adoptiug Claus' Classification^, we create a 

 family of Myzostomidae^ which at present only includes the genus Jf?/- 

 205^0/^?« with its numerous, aboutSO, species. The characters of the 

 family being the following : Parasitic Annelids, mostly hermaphrodite, 

 but some few species unisexual. Body flattened and disc-like with 

 short cirri on the margins. Five pairs of legs , converted into hook- 

 like organs. Proboscis and branched alimeutary canal. Segmental or- 

 gans only represented in the ducts of the reproductive organs. Deve- 

 lopment indirect with larvai metamorphosis. 



' I hope soon to settle this and some other points more definitely. 

 2 1. c. p. 243. 3 1. c. p. 498. 



