[75] NOTES ON ENTOZOA OF MARINE FISHES. 793 



A transverse section of the neck near the head presents a highly 

 characteristic appearance. The outer part consists of two thin but 

 sharply-defined layers. The outer or cuticular layer is made up, in 

 part, of circular and possibly of longitudinal fibers. The inner layer is 

 also a layer of circular fibers. Within this is a somewhat indefinite, 

 narrow region of granular material. Within this again is a very thick 

 coat of longitudinal muscles surrounding a central space which contains 

 the aquiferous and nervous vessels. These longitudinal muscles are 

 collected into broad fascicles, placed side by side and 'standing radially 

 around the central space. This coat is from .05 to .OG" 1 " 1 thick. These 

 dimensions represent the breadth of the muscle fascicles. The thick- 

 ness of the latter is from .008 to .016 mm . These masses of muscular 

 tissue, although parallel, do not yield sections with symmetrical sides. 

 They have, in fact, a crinkled or folded outline. The appearance of 

 central vessels, noted in the neck of the living worm, is thus clearly ex. 

 plained. That appearance is caused by these bundles of longitudinal 

 muscles. In the center of the neck they would, of course, be seen in 

 the direction of their greatest diameter, and would therefore appear 

 more opaque than the surrounding tissues. They would not be defined 

 towards the margins, because there they would be seen in the direction 

 of their least diameters, and moreover several lying in the same enfilad- 

 ing line of vision, they would therefore appear homogeneous. 



The central space, in transverse sections, appears as two oval spaces 

 lying toward the margins and connected at the center by a very narrow 

 line, where the opposite lateral sides of the longitudinal muscle layer 

 almost meet. In each marginal compartment of this central space lie 

 the two aquiferous vessels and another, which I take to be a nervous 

 vessel. Of the two aquiferous vessels, the one in each pair which is the 

 nearer to the center of the neck is the larger. Each is provided with 

 a wall .O03' um thick, which is very sharply defined from the surround- 

 ing granular tissue. The cross-sections of these tubes are oval, and yield 

 the following measurements : Larger vessels, longer diameter, .027 111 " 1 ; 

 shorter, .019 mm ; smaller vessels, longer diameter, .015 mm ; shorter, .012""". 

 The longer diameters of these sections nearly coincide with the breadth 

 of the longer diameter of the neck. The measurements given above 

 include the walls of the tubes. 



Lying close to the marginal side of each pair of aquiferous tubes is 

 another vessel, which I take to represent the nervous system. In 

 transverse sections of the neck, stained with carmine, these appear at 

 first as circular and later as oval patches, which are plainly differen- 

 tiated from the surrounding tissue, but are destitute of the thick limit- 

 ing walls which characterize the aquiferous tubes. These nervous chan- 

 nels are filled with a fine granular tissue, which is but little affected by 

 the staining fluid, although the surrounding tissues are, without excep- 

 tion stained deeply. Where first observed, at the base of the head, the 

 cross-sections of these nervous vessels was circular and .02 inm in diam- 



