344 



MYZOSTOMARIA 



CHAP. XII 



uterus passes backwards above the rectum, and opens either into 

 it or by an independent pore dorsal to the anus. The " lateral 

 oviducts " of Nansen are nephridia with ciliated funnels opening 

 into the coelom (uterus), and with pores leading into the cloaca 

 on its ventral surface ; or, in M. belli Wheeler, opening to the 

 exterior. The two testes are branched, and each sperm -duct 

 opens laterally on a papilla, just outside the third parapodium of 

 each side. Wheeler 1 has recently shown that in the young 

 Myzostoma the spermatozoa ripen before the ova, so that it is 

 functionally a male ; before the spermatozoa are all discharged 

 the ova mature, and the animal is for a time hermaphrodite ; 

 later on, however, when all the spermatozoa are used up, the 

 worm is a female. Beard's " dwarf males " are therefore merely 

 the young of hermaphrodite forms. In cysticolous species each 

 cyst usually contains a large female individual and a small 

 male. In these cases the young one (male) discharges all its 

 spermatozoa before the ova ripen, so that a period of immaturity 

 intervenes and a true hermaphrodite condition is omitted ; the 

 animal is at first male, and later female. The Myzostomaria 

 are thus " protandric hermaphrodites." 



The affinity of these animals has been much discussed ; they 

 superficially resemble the Tardigrada in many anatomical features, 

 and differ greatly from Chaetopoda, but as they possess the 

 characteristic chaetae or parapodia, and pass through a larval 

 stage 2 similar to that of the Polychaetes, there is no doubt that 

 they are closely allied to the group, and indeed may be regarded 

 as degenerate Chaetopods. It has been suggested that they form 

 a passage group between them and the Tardigrada ; and von 

 Graff forms a group Stelechopoda, to include the Myzostomaria, 

 the Tardigrada, and the Linguatulida. 



1 ML Zool. Stat. Neapel, xii. 1896, p. 227 ; where, too, see literature. 

 2 Beard, ML Zool. St. Neap. v. 1884, p. 544. 



