300 F. F. LAIDLAW. 



the inner pharyngeal epithelium (i. e.) is a thick inner circular muscle layer (t. c.) followed 

 by a narrower inner longitudinal layer (i.L). Between this latter and the median longitudinal, 

 or retractor muscle layer (m. I.) lie the salivary glands {sa. gl.), occupying fully one-half the 

 width of the proboscis. Whilst passing through these the radial muscle-fibres are gathered up 

 into more or less compact bundles. 



Gut. The gut-diverticula are precisely similar in structure and arrangement to those of 

 P. siphunculus. Their walls are crowded with exceedingly minute bacteria-like structures, which 

 are entirely absent from other parts of the body. In the main-gut, 



however, I have found what appears to be a distinct kind of bacterial ^Mj^ M^ _ 



organism in various stages of growth (Fig. 71). The smallest, pre- ^^^ ^i|^ ^ \ 



sumably the youngest stages, are rod-like; as the organism increases ^j^ 7^ Bacteria-like oiRanism 

 in size it gradually acquires a dumb-bell appearance, until finally it from the gut of Prostkiosto- 

 consists of two nearly equal sub-spherical masses united by a short mum ehgans. 

 stalk. Its subsequent fate I have not been able to trace. 



Genital apparatus. Of the female organs only the vagina and shell-glands are de- 

 veloped ; they are precisely similar to those of P. siphunculus, and in fact of most Cotylea. 

 The male organs are in full functional activity. In this species the relative size of the ter- 

 minal parts of the male apparatus is much greater than in P. siphunculus, otherwise they are 

 very similar in the two species. 



The two large vasa deferentia run inwards and forwards at their distal ends to open into 

 the vesicula seminalis, a large sac with muscular walls, composed of circular fibres, lined on 

 the inside with a flattened epithelium. The vasa deferentia do not open into the extreme 

 hind end of the vesicula, but pass into its walls on either side a short way in front of its 

 hinder extremity, and then run backwards in its walls for a short distance before opening 

 into it. This arrangement may act as a valve to prevent 'regurgitation' of the sperm, when the 

 vesicula is contracted suddenly. From the anterior end of the vesicle a ductus ejaculatorius runs 

 forward to enter the penis. It is lined on the inside with a flattened non-ciliated epithelium, 

 outside which is a very thin layer of circular fibres. 



Immediately in front of the vesicula, lying side by side above the ductus, are two large 

 spherical vesicles with thick muscular walls composed of circular fibres; outside the muscular 

 layer is a sheath of specialized parenchyma, rather denser than the ordinary tissue. These 

 vesicles are called by Lang 'accessory vesicles.' Their lumen, which is fairly spacious, is 

 lined with a flattened epithelium, bearing a few cilia. Each is provided with a short duct 

 — relatively much shorter than in P. siphunculus — which runs forwards, twisting greatly as 

 it goes, lying for the most part dorsal to the ductus ejaculatorius. These accessory ducts, as 

 they may be called, differ greatly from the latter. They are in the first place much narrower, 

 they have a relatively thicker outside wall of circular fibres, and the epithelium through 

 which they run is tubular, that is to say the ducts are intracellular. They pierce right through 

 the muscular coats of the accessory vesicles. " Sie sind innen von einem verschwindend nie- 

 drigen Plattepithel ausgekleidet, in welchem die Zahl der einzelnen Epithelzellen andeutenden 

 Kerne so gering ist, dass auf einen Querschnitt des Canales nie mehr als ein Kem zu liegen 

 kommt, so dass man hier auch von durchbohrten Zellen sprechen kann." (Lang [9], p. 27(3.) 



The ductus ejaculatorius (which for convenience I will simply call the ductus) as it ap- 

 proaches the base of the penis is joined by the two accessory ducts, which enter its walls and 

 run forward in them for some distance, lying immediately to the outside of the epithelium of 



