SPECIAL ANATOMY. 71 



The blood contains numerous blood-corpuscles, which vary in size, are 

 granular, and exhibit numerous radiating, projecting points of variable 

 length, — frequently greater than the diameter of the corpuscle. 



The pulmonary cavity is lined with a tessellated epithelium, the cells 

 of which are faintly granular, with a few coarser granules, and a distinct, 

 round or oval, granular, nucleolated nucleus. 



The renal organ is a gland which exhibits a foliated or plicated ap- 

 pearance, within a capsule. The surfaces of the plicae communicate 

 with the duct existing along the whole right border of the organ, and 

 are covered with polygonal, oi-ganic cells, every one of which contains a 

 large, white, round, opaque mass, resembling uric acid in appearance. 



On the Nervous Centres, and the Distribution of the Nerves. 



The nervous centres consist of three distinct sets of ganglia, which 

 are all placed within the anterior part of the body or head. 



The first set, or supra-oesophageal ganglia, form a transverse band, 

 above or in front of the buccal body, usually at its anterior part, but 

 varying in the latter position, to some degree depending upon the 

 movements of the buccal body. When the latter is protruded, the band 

 of ganglia is thrown back to the commencement of the oesophagus ; 

 when retracted, it is placed just behind the upper lip. It consists of 

 two symmetrical halves united by a short transverse commissure. Each 

 half is composed of several ganglia, aggregated to a greater or less de- 

 gree in different genera and species. Sometimes they are so aggregated, 

 or are so covered by enveloping tissue, as to appear a single mass ; in 

 others five or six distinct masses may be readily counted. 



The second set, or sub-oesophageal ganglia, form a circular mass, 

 placed infero -posteriorly to the buccal body, in the excavation of the 

 foot. It is asymmetrical, and is composed of several ganglia, more or 

 less aggregated together ; from four to seven masses can generally be 

 counted. It is usually more developed upon the right than the left 

 side, and passes through all the shades of color, in different genera and 

 species, from white and yellow to orange. Through the opening formed 

 by the arrangement of the ganglia into a circle passes the cephalic 

 branch of the aorta. The supra-cesophageal and sub-oesophageal ganglia 

 are connected together on each side of the buccal body by a double 

 commissure, which varies in length in different genera ; thus, in Helices, 

 etc., it is generally so long as to allow of much movement of the supra- 

 cesophageal ganglia forward with the buccal body, while in Veronicella 



