No. 2.] STRUCTURE OF BIMASTOS PALUSTRIS. 48 3 



The prostate is very well developed and to its large size is 

 due, in a great measure, the conspicuous swelling noticeable in 

 the region of the genital pores. It occupies somites XV and 

 XVI, being strongly constricted into two lobes by the inter- 

 vening septum. The portion lying in somite XV is by far the 

 larger, and almost fills that somite on each side of the alimen- 

 tary canal. 



The lobe in somite XVI is shorter and less robust, being 

 confined to the anterior portion of the somite. 



The lumen of the prostate lies entirely within the lobe 

 occupying somite XV; dorsally, where it receives the sperm- 

 duct, it is quite narrow, but in its ventral two-thirds it rapidly 

 enlarges and extends laterally to the external pore already 

 described. In section the cavity is somewhat triangular in shape, 

 as may be seen in Figs. 13 and 14. Fig. 13 is a longitudinal 

 section near the entrance of the sperm-duct, and Fig. 14 is 

 taken from a section, external to this, through the male ex- 

 ternal pore. It is lined by non-ciliated columnar cells, inter- 

 spersed with the necks of other cells which may be distinctly 

 recognized as glandular. In some regions the columnar cells 

 form a true lining epithelium, but in most places they are 

 numerically of secondary importance. Where they are most 

 abundant their outlines are quite distinct, and each is seen to 

 contain an oval nucleus and a nucleolus. 



Externally to the columnar cells is a thick wall of club- 

 shaped unicellular glands, differing in appearance in different 

 regions. The prostate is well supplied with blood-vessels ; the 

 muscular portion is poorly developed, and instead of forming a 

 distinct layer, as in Moniligaster, the fibres are irregularly dis- 

 tributed amongst the gland cells. 



Upon its coelomic surface it is covered by a delicate coelomic 

 membrane, continuous with that lining the coelom generally, 

 and dipping down between the imperfect lobules into which 

 the gland-cells tend to arrange themselves. 



In the upper two-thirds of the prostate, these glandular cells 

 have clear, almost transparent, oval heads with central nucleus 

 and nucleolus, and long duct-like necks which run in a mass 

 towards the central lumen. The swollen nucleated portions 



