26 
THE VOYAGE OF H.M.S. CHALLENGER. 
and some an unlimited number ; the former appear to me to be the most specialised forms. 
The Myzostomida Caudata with the caudal appendages have originated from discoid 
forms (see p. 8), in which some of the cirri have taken on a special development, 
whereas the Myzostomida Cysticola would seem to be retrograde forms, from the absence 
of suckers and the rudimentary radially arranged parapodia, as well as from the presence 
of muscular septa and the central convergence of the parapodial muscles. They have 
probably originated partly from the stout sessile species grid partly from the more delicate 
free living species, as shown by the fact that the cirri are sometimes entirely absent 
(■ Myzostoma cysticolum), and sometimes, on the other hand, highly developed ( Myzostoma 
willemoesii) in both sexes. The last-mentioned species is also remarkable from the fact 
that both sexes possess well-developed suckers, and shows therefore a less degree of 
degeneration than the other cysticolous forms ; also the male has the typical ramified form 
of the testis found in the free living species, and not the compact rounded organ of the 
other Myzostomida Cysticola. 
