Note Oli the stvucture and relations of the kidney in Aplysia. 425 



deeply stained lines forming- a network witbout any regulär arraugemeut 

 and on eaeli side of tliese lines is seen the renai epitbelium. In the upper 

 part of fig. 3 the renai epithelium is not drawn. The smaller spaces 

 enclosed by the trabeculae in the seetions are filled completely by the 

 outlines of tbe vesicular renai cells. This seems to be due to the continuai 

 proliferation of the cells of the renai epithelium, the escape of the mature 

 cells being impeded by the narrowness of the intertrabecular spaces. 

 The trabeculae are in many places extremely thin and no blood lacunae 

 can be detected in these, the epithelium covering them deriving its nou- 

 rishment from the blood, which they absorb from neighbouring lacunae : 

 probably some lacunae, wich exist in the living animai, are obliterated 

 by the contraction, which takes place during the process of preparation. 

 But a great many of the trabeculae are hollow consisting of two mem- 

 branes enclosing a blood lacuna between them ; in the seetions such 

 blood lacunae may be seen extending along the trabeculae for some 

 distance and containing blood corpuscles; in some places, especially 

 where the trabeculae take their origin from the side of the renai cavity 

 towards the wall of the branchial Chamber, there are blood lacunae of 

 considerable size traversed by delicate Strands of connective tissue 

 (fig. 3 hP). In fig. 5 a portion of a trabecula with a blood lacuna run- 

 ning along it axially is figured as seen under high power ; it is taken 

 from a section from the posterior part of the organ. Muscular tissue is 

 present in the trabeculae bere and there , but not in any great quanti ty. 

 An example is given in fig. 6, where the centrai part of the trabecula is 

 occupied by a band of parallel muscular fibres : the renai epithelium in 

 this particular part of the section was not well preserved. 



The renal epithelium of Aplysia seems somewhat different at first 

 sight to that which is characteristic of most Gasteropoda : but it has the 

 same fundamental structure. When a portion of the kidney in the fresh 

 state is teased up on a slide and examìned under the microscope, the 

 field of view is seen covered with vesicular cells of various sizes contain- 

 ing concretions. Most of the cells are quite free, and sphericalin shape; 

 in some of them a layer of transparent protoplasm extends round part 

 of the circumference, but no nucleus can as a rule be distinguished even 

 after the addition of acetic acid ; sometimes as in / fig. 7 an elliptical 

 body is seen in the protoplasm, which may be a nucleus, but the whole 

 is so transparent, that its nature cannot be distinctly made out. In see- 

 tions large nuclei are seen belonging to the cells of the renai epithelium 

 as in fig. 5. In a teased preparation the upper surface of the epithelium 

 is often seen, slightly pressed by the cover glass, and the appearance it 



28* 



