THE MARINE TUKBELLARIA. 285 



and an inner circular layer. At its anterior proximal end it is pierced by the ductus ejacu- 

 latorius running into it from the small muscular vesicula seminalis {v. s.). At the same end 

 lies the large prostate gland (pr.), whose duct joins the ductus ejaculatorius some way before 

 entering the penis. Retractor muscles run from the upper end of the penis, and from the 

 distal jjart of the ductus to join the outer sheath. 



A transverse section through the distal lower limit of the penis is represented somewhat 

 diagrammatically in fig. 13. The irregular lumen is seen to be lined with short chitinous 

 spines (c. s.) and the section passes through the bases of the six large hook-like structures (c. h.). 

 Outside the spines and hooks is a narrow layer of hyaline protoplasm, and beyond this the 

 irregular, diagonal, intrinsic muscles of the penis (p. ni.) in close contact with the muscles of 

 the outer sheath, here rather feebly developed. The small cavities (a. m.) are parts of the 

 antrum masculinum. The irregularity of the lumen is probably due to shrinkage. Further 

 forward a transverse section shows the hyaline layer rather more clearly defined, and outside 

 it is a broad muscle layer composed of diagonal fibres which are closely packed together on 

 the inside, but loosely arranged to the outside, so that no very clear boundary can be distin- 

 guished between the muscles and the parenchyma, which have divided the intrinsic muscles of 

 the penis fi'om those of the outer sheath. 



At the level of the chitinous ' collar-like ' structure the sections have unavoidably been 

 torn. Consequently I cannot be certain of the relationship of the collar to the penis or to 

 the retractor muscles, but some of the latter seem to be attached to it. The collar itself, 

 seen in the animal when cleared in oil of cloves before it was cut into sections, appeared 

 as a ring of chitinous material, its uf)per, anterior end wider than the lower, and its outer 

 surface decorated with a rectangular sculpturing. On account of the tearing of the sections 

 it is also impossible to say whether the ductus ejaculatorius entered the penis by passing 

 through the collar or to the outside of it. 



In a transverse section in front of the penis but before the level of the prostate, the 

 ductus ejaculatoi'ius can be seen running through the middle of the section. It has a narrow 

 lumen lined with ciliated epithelium. Outside this is a thin layer of circular muscle fibres 

 followed by a longitudinal layer. From this longitudinal layer a number of bundles of 

 retractor muscles are given off, exactly similar to those connected with the hinder end of the 

 penis itself In sections at this level many of them lie cut across obliquely amongst the 

 parenchyma which here fills the wide space between the ductus and the outer sheath. The 

 ductus runs nearly straight back to the prostate. Just before entering the latter it divides 

 into a wider part running to the gland and a narrower part continuing to the vesicula 

 seminalis, after piercing the wall of the outer sheath. It is interesting to find here a 

 little peculiarity which evidently also occurs in PI. pellucida (see von Graff's figure [6]), 

 i.e. the duct fi-om the vesicula does not open immediately into that fi-om the prostate, but 

 is actually prolonged for a short distance inside it. This prolongation is probably of the nature 

 of a valve. Each of these two parts making up the ductus ejaculatorius is provided only 

 with circular muscle fibres; but, whereas those of the prostatic part are few in number, those 

 of the part running to the vesicula though few at first increase rapidly in number as the 

 vesicula is approached. 



The prostate (pr.) is a large gland, lying in close contact with the outer sheath at its 

 anterior end ; in fact the circular muscles of the sheath completely enfold it, and form a muscular 

 wall for it. On the inside it is lined with a much-folded glandular epithelium. The vesicula 



