54 TBLETHUS^. 



differences, however, exist in the appearances of the parts in preparations, apparently 

 from the methods adopted. 1 When sections pass through the heart-bodies, their structure 

 is as described by Dr. Ash worth. 2 



In transverse section of a rigidly contracted example the nerve-cords are pyriform, 

 the elongated stalk bifurcating as it approaches the circular muscular coat and the fibres 

 enter it. These nerve-cords have been thrust inward by the close approximation of the 

 ventral longitudinal muscles, and the neurilemma has been elongated into a stalk, a con- 

 dition which might mislead during a cursory examination. 3 Cunningham states that a 

 small neural canal occurs at the dorsal and inner side of each cord. The pyriform nerve- 

 area is partly divided by a granular band from the stalk. The inner edge of the great 

 longitudinal muscles is covered by a distinct sarcolemma. Traces of an oblique muscle 

 appear between the second muscular wedge of the longitudinal layer and that above it. 

 A fissure occurs in the mid-dorsal line, where the longitudinal coat is thinner, and on each 

 side muscular wedges (in section) occur above the traces of the oblique. In the anterior 

 region the oesophagus has a thin investment, probably of both circular and longitudinal 

 fibres, and its mucous surface is richly glandular. The oesophageal sacs or glands have a 

 thin wall of similar structure, but their mucous lining forms deep folds, which in trans- 

 verse section present a somewhat pinnate appearance from the cells and their nuclei. 



In transverse sections of the posterior region of A. marina toward the tip of the tail, 

 the circular muscular layer is proportionally strong, and the longitudinal muscles of con- 

 siderable thickness. The nerve-area has the same position — inside the circular muscular 

 coat, but the trunks are more distinct than in front. The neural canal is still at the 

 upper border, and, indeed, separates the nerve- cords superiorly, and they often present 

 a slight obliquity — that is, they are slightly tilted outward superiorly. Externally is a 

 thick layer of neuroglia, the cells of which stain deeply, and pass upward between the 

 cords like a wedge, then nerve-sheath, basement tissue, hypoderm, and cuticle. The 

 oblique muscles go to the upper and outer angle of the nerve-area, and a few fibres pass 

 down its side to reach the circular. The gut is wide, but agrees in structure with that in 

 front. No apparent differentiation of the cords into ganglionic or inter-ganglionic regions 

 occurs either here or in front. 



In Arenicola ecavdata the nerve-trunks are thrust further inward than in A. marina, 

 but the oblique muscles preserve the same relations. A single neural canal occurs at 

 the centre superiorly, in the anterior region, but by-and-by two larger canals appear. 

 On the whole the cords in section are more pyriform than in A. marina, and are fixed by 

 neurilemma to the circular muscular layer. The cells of the neuroglia form two bands 

 inferiorly, a space existing between them. In a preparation of Dr. Ashworth's the 

 sperm-cells occur in great lobular masses in the coelom. 



In Arenicola branchialis the arrangement of the layers of the body-wall is typical, 

 and the outline of the nerve-trunks in section conforms rather to that in A. marina than 

 in A. ecavdata, the shape being ovoid, with a mass of neuroglia inferiorly and strands of 



i ( Quart. Journ. Micr. Sci./ vol. xxviii, n.s., p. 273. 



2 ' L. M. B. C. Memoirs/ xi, p. 35, pi. v. 



3 I am indebted to Dr. J. H. Ashworth for kindly sending slides of the sections of A. marina, 

 A. branchialis, and A. ecaudata for comparison with my own, some of which were made by the hand. 



