230 PROF. W. A. HASWELL : 



The structure of the muscular organ which I have called the gizzard, but 

 for which the name proventriculus used by Malaquin and others will be 

 employed here, is similar in all essential respects to that of the corresponding 

 organ in the Syllida?. The proventriculus is a thick-walled cylinder, the 

 lumen of which is reduced in the passive condition of the organ to the form 

 of a vertical slit. Along the dorsal and ventral median lines opposite the 

 ends of the slit run dorsal and ventral raphes along which the thickness of 

 the musculature is greatly reduced. The greater part of the thickness of 

 the wall is taken up by radiating muscular columns arranged in annular 

 rows. 



Malaquin (p. 213) states that the wall of the proventriculus consists of the 

 following layers : — (1) peritoneal layer ; (2) external circular muscles 

 forming a thin layer ; (3) radial muscle-columns separated by transverse 

 diaphragms ; (4j internal circular fibres; (5) the general columnar epi- 

 thelium of the digestive tube ; (6) the cuticle. 



How far this is correct in relation to the Syllidea, Eusyllidea, and Auto- 

 lytidea will not be discussed in this paper. In Exogone fustifera (and the 

 other Exogonece which I have studied) there are no layers of circular muscle 

 — the only circular fibres being those in the so-called diaphragms. 



That the radial muscle-columns are in the Syllidse striated muscle-fibres 

 of a primitive type was pointed out by me (16) in 1886, and the subject was 

 further elaboratt-d in 1889 (17). 



Malaquin (22), who was unacquainted with my second contribution to the 

 subject, arrived independently at very similar results. 



In Exogone fustifera, as in all the other Exogonece which I have examined, 

 the muscle-columns (PI. 18. figs. 37, 39 & 40) are non-striated : they contain 

 no doubly refracting substance and are devoid of transverse networks. 

 In other respects they closely resemble the muscle-columns of Syllidea. 

 The}' have a cortex of muscle-substance made up of fibrilla?, and a core of a 

 granular protoplasmic material. Near the squarish outer end of the column is 

 a nucleus usually single in the middle of the core ; and in this region the 

 protoplasm of each muscle-cell communicates with that of its neighbours in 

 the same row by means of narrow processes. 



In addition to these peculiar muscle-columns with their protoplasmic cores, 

 the wall of the proventriculus also contains muscular fibres of the same 

 character as those occurring in other parts of the body. These are arranged 

 in narrow annular bands corresponding to the annular rows of muscle- 

 columns. Each of these in Exogone fustifera (PI. 18. figs. 39 & 40) runs 

 through the muscle-columns of the corresponding annular row close to then- 

 outer euds. 



The muscle-columns of E. fustifera are divided by the circular muscles, 



