ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 67 



certain regions of the body. On tlie apex of the prsBoral lobe is a crown 

 of stiff long cilia which do not represent the prgeoral ring of cilia 

 characteristic of annelidan larvae ; there is a well-developed postoral 

 and a preanal ring as well as a tuft of stiffish cilia on the postanal 

 papilla. At the same time the provisional setse make their appear- 

 ance ; on each side of the body behind the mouth is a single seta which 

 grows rapidly, and finally comes to be much longer than the body 

 itself ; subsequently to the third day the number of setaB is increased, 

 but they are never arranged metamerically as e.g. in Nereis dumerilii ; 

 muscles are developed in connection with these sette. There appear 

 to be no larval segmental organs, but this fact does not militate 

 against the affinities of the group for the Chsetopoda, inasmuch as 

 Nereis dumerilii is similarly without a larval nephridium ; an embry- 

 onic nervous system is found in the praeoral lobe, and traces of the 

 future ventral cord are observable in this stage. 



Third period. — The provisional setae are cast off at about the 

 seventh day, and the larva moves about with a worm-like motion ; in 

 no case were the parapodia developed in the free larvas, and in the 

 absence of any organs of locomotion it is difficult to understand how 

 the larva finds its way to its host —the Comatula ; the probability 

 seems to be that the currents of water produced by the waving arms 

 of the Comatula bring the parasites. Whatever may be the means by 

 which the larval Myzostoma comes to a secure anchorage on the body 

 of a Comatula, it undergoes fresh changes as soon as it arrives at that 

 situation. The cuticle becomes thicker, the cilia spread over the 

 whole surface of the body arranged in tufts; the parapodia are 

 formed from before backwards. The functional nervous system of 

 the larva contained within the prseoral lobe entirely vanishes in the 

 adult, where there is no supra-oesophageal ganglion ; in the adult the 

 nervous system consists only of a ventral mass which is formed by 

 the fusion of several ganglia. 



All the developmental facts appear to be in favour of regarding 

 Myzostoma as a Ohaetopod degenerate through parasitism ; the 

 larva with its ciliated bands and provisional setee can be compared 

 to nothing else. 



The paper concludes with some notes on the biology of the 

 genus. 



In M. glahrum during the winter months a number of small 

 specimens were observed by Graff attached to the back of an adult, 

 but it appears that these individuals are not immature as Graff 

 supposed, but fully developed males with functional spermatozoa; 

 this species therefore presents another instance of the remarkable 

 phenomenon first described by Darwin in the Cirripedia, viz. the 

 occurrence of " complemental males " ; these small males resemble in 

 all respects, except in that they possess no trace of female organs of 

 generation, the hermaphrodi ,es upon which they sit. Among the 

 Cirripedia Darwin discovered all stages from unisexual forms to 

 complete hermaphrodites ; and the same is the case with the 

 Myzostomidse as recently proved by Graff from an examination of the 

 ' Challenger ' collection. It is of great interest to consider which is 



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