508 JOSEPH STAFFORD, 



outwards and were lost ou the outer surface of the animal. In the 

 living animal so compressed that these were turned upwards they 

 were closed and half-moou-shaped, curving towards one another with 

 a circular disk of the body-surface lying between them. This disk 

 was continuous above and below with the surface surrounding them 

 in which could be recognized circular fibres that acted as constrictors. 

 While not under too much pressure these excretory pores were con- 

 tinually changing — their walls first contracting, then expanding, now 

 their margins were rolled farther inwards, then forced out again. I 

 have seen when they were so far extruded as to form two short 

 siphons. It is also possible for them to be drawn inwards far enough 

 for to leave only a single outwardly opening conical depression of 

 the outer surface at the bottom of which are the real excretory pores. 

 This happens when the posterior part of the body is stretched far 

 backwards but the foot projected forwards so as to put the large 

 expulsion canals, continuous with the pores, and their surrounding 

 tissues in tension. In horizontal sections each porus expands inwards 

 into a large cavity that narrows, as it passes forwards, downwards, 

 and outwards, into the large expulsion canals of the foot. But a 

 short space from the pori excretorii these two canals communicate by 

 a wide transverse canal (Fig. 23, 33 TO), or rather their inner walls 

 disappear and the cavities of the two coalesce. This is the only 

 connection between the two systems — that of the left and that of 

 the right side — there being no anastomoses of the vessels as Pagen- 

 STEGHER (1857) thought. The transverse communication separates otf 

 a small portion of body parenchyma (Fig. 23, 33 P) lying behind it 

 and between the backwardly extending pores but continuous above 

 and below with the parenchyma surrounding these vessels. It thus 

 forms a narrow vertical partition between the foramina caudalia. 



The cuticle of the external surface is continued into the excretory 

 pores at least 60 i^i and where it ceases a layer of epithelium cells 

 with large nuclei begins. Where the large nucleolated nuclei are at 

 a distance from one another the layer in which they lie can be seen 

 to considerably resemble the cuticle in depth and general appearance. 

 It does not show distinct cell boundaries. Outside of the epithel- 

 cells are difficultly discoverable longitudinal and much thicker circular 

 muscle fibres. I have only seen these to advantage in sections cut 

 lengthways with the vessels where a portion of the wall lies uncut 

 showing surface view. 



The expulsion tubes (collecting vessels) (Fig. 11, 12, 2, 7 CV) 



