96 DR. C. L BOULENGER ON 



of the small nerves, at least in the species which he examined, 

 M. asterice Mai-enz. 



I spent some time in attempting to reconstruct the nervous 

 system of M. costatum, as I hoped that the distribution of the 

 paired nerves might shed some light on the problem of the homo- 

 logies of the additional suckers in this species ; the diagram (text- 

 fig. 20) on p. 95 gives, I think, a tolerably accurate idea of the 

 nerves arising from the ventral nerve-mass. 



It will be seen from the diagram that the nervous system of 

 M. costatum is remarkably similar to that described by Nansen 

 in other species, and especially to that of M. cirriferuvi ; the 

 number of nerves arising from the central mass is normal ; there 

 are eleven pairs, of which five are large and branch chiefly to the 

 parapodia. I agree with v. Stummer in believing the suckers to 

 be supplied by branches of the smaller intermediate nerves ; the 

 six paii^s run almost straight out towards the six pairs of suckers, 

 but I was unable to trace the finer branches actually into these 

 organs. 



It will be seen from my diagram that in M. costatum the origin 

 of the six pairs of smaller nerves differs somewhat from that in 

 the species described by ISansen ; the first on each side seems to 

 originate fi'om the circumpharyngeal commissure, whilst the re- 

 maining five are given off posteriorly from the bases of the five 

 larger nerves to the parapodia. 



If we take for granted that the suckers in Myzostoma are 

 " Seitenorgane " homologous with those of other Annelids and 

 therefore parts of the parapodia — and I think "Wheeler and 

 V. Stummer have made out a very strong case for such a 

 homology, — then it becomes obvious from the above account that 

 each sucker belongs to the parapodium anterior to it. In the 

 majority of species the fifth pair of parapodia has lost its 

 " Seitenorgane," these being retained in four species only — ■ 

 M. astericp, M. costatum, M. mcehianiom* , and Protomyzostomimi 

 polynephris. In the latter form they have remained in the more 

 normal position, i. e., lateral and dorsal to their respective para- 

 podia. The innervation of the first of the six pairs of suckers in 

 M. costatum from the circumpharyngeal commissures suggests 

 that these may belong to a peristomial segment from which, as 

 in other Annelids, the locomotory portions of the parapodia have 

 disappeared. 



I wish again to emphasize the fact that I have been unable to 

 prove that the suckers really are innervated from the six pairs of 

 finer nerves, and that therefore the above conclusions are highly 

 speculative. I think, however, that they afford a satisfactory 

 explanation of the variation in the number of suckers in this 

 group. 



Sexual Organs. 



Like nearly all well-investigated species of Myzostoma, M. cos- 

 tatum is hermaphrodite, and all the larger specimens in 



* Cf. Bouleng-ev (3, ]). 350). 



