92 DR. C. L. BOULENGER ON 



accepted views on the morpliology of the M3'zostomida. W. M. 

 Wheeler, in his important paper on the sexual phases of these 

 animals (17), discusses the structure and homologies of the suckers 

 at some length. These organs (which he prefeisto call segmental 

 sacs) he considers to be metameric, lying laterally or dorsally 

 to their respective parapodia, like the " Seitenoi-gane " of the 

 Capitellidse, and for various i-easons he believes them to be 

 homologues of these lateral line organs or segmental sacs. In 

 the course of his discussion of this subject we lind the following- 

 passage : — " The fact that in Myzosioma there are five pairs of 

 parapodia but only four pairs of segmental sacs, natuially leads to 

 the question as to what has become of the missing pair of sacs. 

 The answer to this question I believe we need not go far to seek ; 

 the third ijair of the origiiial Jive 2^(t,'>rs of sacs has been converted 

 into the so-called jjenes. These are more or less prominent 

 papillae, lateral to and near the bases or the third pair of para- 

 podia. Each papilla is perforated by a ductus ejaculatorius, which 

 widens proximally into a vesicula seminalis. The latter receives 

 the mature spermatozoa from the vasa deferentia, and these in 

 turn from the ramifying testicular follicles. Bizarre as the de- 

 velopment of a male reprodu.ctive organ from a lateral line organ 

 may appear at first sight, I am nevertheless unable to see any great 

 difficulty in such a change of function. It is, in fact, easy to see 

 how the bottom of an eversible sac might acquire an opening into 

 the body-cavity under the pressure of a great accumulation of 

 spei'inatozoa ; the sac would then become reduced to a. mere 

 conduit." 



A few years later von Stummer-Traunfels (16) investigated the 

 anatomy of the curious endoparasitic form JI. asterice Marenz., 

 and showed that this species possesses a median sucker behind the 

 last pair of parapodia ; whilst describing Wheeler's view of the 

 homology of the penes as somewhat far-fetched, he considers this 

 extra sucker to represent the missing fifth pair, and his researches 

 on the innervation of this organ show that it must have been 

 formed by the fusion of two originally separate suckers. 



Since the publication of my preliminary note (3) on the suckers 

 of M. costatiovi, Fedotov (6) has described a remarkable type of 

 Myzostomid, an endoparasite of the brittle-sta,r Gorgonocejihalus 

 eucnemis Miiller et Troschel ; in this form, named Protomyzo- 

 stomum polynephris, five pairs of suckers are present, and these 

 are not situated between the pairs of parapodia on the ventral 

 side, but opposite to them and in a dorsal position. It is to be 

 hoped that further investigation of the anatomy of this new genus 

 may throw some light on the relations between suckers and 

 parapodia in the Myzostomids ; meanwhile, the occurrence of six 

 pairs of suckers in M. costatum merely complicates the problem. 

 Granting the metameric nature of these organs, we must now 

 consider that we are dealing with six segments instead of five, 

 and it seems necessary to account for a missing sixth pair of 

 parapodia *. 



* Cf. also remarks under '" 2\crvous 83>tem " on p. 95. 



