On the Life-History and Development of the Genus Myzostoma. 565 



some such as M. cijsticohim etc. are bisexual. Neither are the sexual 

 Organs built on the same plan in both groups. In the Tardigrada the 

 ovary is a small well defined compact organ producing a few large 

 eggs, according to Kaufmann seven or eight in number. In Myzostoma 

 it is not a compact organ , but is scattered throughout the body and 

 produces great numbers of minute eggs. The complicated male organs 

 of the Tardigrada to an even less degree show resemblances to those 

 of Myzostoma. In the latter the legs are not built on the same plan and 

 are not equal in number to those of Tardigrada. The Myzostomidae are 

 ciliated, the Tardigrada not. 



But if all anatomical likenesses were granted there would still re- 

 main the insurmountable obstacle of the development. The larva of 

 Myzostoma with provisionai setae is undoubtedly allied to the Chaeto- 

 pods. Indeed the only group to which it can belong is the Chaetopoda. 

 If we compare it with Götte's larva of Nereis Dumerilii , a rough copy 

 of which is given in fig. 29, the resemblances will be obvious enough. 

 Both have a praeanal ring of cilia. Nereis Dumerilii hsiS also a praeoral 

 rmg, while Myzostoma has a postoral. Both have mouth, simple alimen- 

 tary canal and anus. Both have a thickening of the epiblast of the prae- 

 oral lobe which functions as the larvai nervous system. Both have also 

 a similar ventral thickening of the epiblast , which is the rudiment of 

 the future ventral ganglia. 



Both have provisionai setae, the only diflference here being that in 

 Nereis Dumerilii theìsn'YH is segmented, while in Myzo- 

 stoma it only becomes so in the later development. So in 

 Myzostoma the larvai setae are not arranged as in Nereis DuineriUi. 

 The latter possesses an eye , while Myzostoma does not , but then in 

 Myzostoina the region in front of the mouth is somewhat rudimentary 

 and in this rudimentary condition of the praeoral lobe one sees the 

 reason of the main difiference between Myzostoma and more typical 

 Chaetopod larvae. Owing to this too the praeoral ring has disappeared. 



In neither N. Dumerilii nor Myzostoma are segmental organs to be 

 found, and this negative character obtained in both Myzostoina and a 

 true Chaetopod , can furnish no grounds for refusing to classify Myzo- 

 stoma with the Chaetopods. 



As to other resemblances in both N. Dumerilii and Myzostoma the 

 segmentation is complete but unequal , in both it results in an epibolic 

 gastrula , and probably in both the mesoblast arises in the same way. 

 Most , indeed , if not all of the characters of Myzostoma speak for its 

 union with the Chaetopoda. Its development is quite that of a Chaeto- 



