The Sexual Phases of Myzostoma. 
287 
the minute structare of the Myzostome and Chaetopod seta^ Such 
being the case we should expect to find a great similarity in the 
structures which scerete the setae in both cases. It is well known 
from the researches of Leydtg [Phreoryctes '65, pag. 256) ; Claparède 
[Terehella'l^)\ Perrier [Lumhricus '74, pag. 347); Spengel [Echiurus, 
Bonellia '80, pag. 478 — 484); Vejdovsky [Bendrohaena^ Lumhricus etc. 
'84) and EisiG (Capitellidae '87, pag. 575 — 576) that each seta is 
secreted by a single large celi situated in the fundus of the seta-sac. 
In ali Chaetopods, so far as known, this »Basalzelle« has more or 
less the form of a plano-convex lens with its fiat side covering the 
truncated proximal end of the seta which its cytoplasm gradually 
secretes. In Myzostoma also each of the modified setae is se- 
creted by a single large celi, but this celi, instead of hav- 
ing the form of a plano-convex lens, is pear-shaped and hangs by 
a thin pedicel to a much flattened disc-like piate of cytoplasm 
covering the truncated end of the seta. One of these cells is seen 
at st.c in PI. 1 1 Fig. 25. The Constant association of these cells with 
the setae, their large size and characteristic glandulär appearance, 
leaves no doubt that they are the homologues of the »Basalzellen« 
of Chaetopods. These pear-shaped setiparous cells are figured by 
Nansen, and he says at pag. 77 : »1 bave frequently observed at the 
upper extremity of the supporting rods large unipolar cells, with 
their prolongations directed towards the extremity of the rods (PI. 7 
Fig. 22a.5), but I bave been unable to determine in what manner 
these prolongations terminate, or what is their physiological function.« 
Perrier observed in the earthworm ('74, pag. 347) and Eisig 
in Capitellids ('87, pag. 576) certain appearances in the nuclei of the 
basai cells which led them to infer that the nucleus is concerned in 
the process of secreting the seta. The position of the rather small 
nucleus in the broadened bulb-shaped end of the basai celi in 
Myzostoma^ and its distance from the secreting cytoplasmic piate 
does not favor Perrier's and Eisig's view, but a careful study of 
the nucleus and cytoplasm will bave to be undertaken before any 
definite statement can be made on this point. 
6. The streng Chaetopod affinities of Myzostoma are further 
shown by their resemblance to the Polychaeta of the 
genus Spinther. v. Graff has called attention to the resem- 
blances in the closing paragraph of bis interesting monograph of the 
1 The similarity is clearly recognized by Eisia '87, pag. 576. 
