POST-LARVAL STAGE OF ARENICOLA MARINA, 53 
DESCRIPTION OF PLATE I, 
Illustrating Mr. Benham’s paper on “The Post-larval Stage of 
Arenicola marina.” 
Fia. 1.—View of the larva mounted whole (from a camera drawing, under Zeiss aa, 
oce. 2). x 38. The animal is lying on its right side anteriorly, but is twisted in Somite 
13, so that it presents its dorsal surface upwards. The dorsal chet of the right side are 
seen as far forwards as Somite 8. The ventral chat are seen anteriorly, but have not 
been represented after Somite 13. The rudimentary gills are seen in Somites 14 to 21. 
The somites in the “tail” are indistinct except towards the anus. The dark shading 
round the animal represents the structureless envelope secreted by it. Ch. 2. Rudimentary 
dorsal cheta of Somite 2. div. The diverticulum of stomach of the left side. d.ch.l. The 
dorsal chet of the left side. d.ch.r. The dorsal chetw of the right side. int’. Narrow 
intestine. int?. Wide intestine. m. Mouth. q@s. (sophagus. ofo. Area deprived of 
pigment, below which is situated the otocyst. Pros. Prostomium. sac. Sacculated region 
of intestine. stom. Stomach. 
Fia. 2.—One of the ventral chetz from a transverse section. (Camera, Zeiss D. 4.) 
x 500. 
Fig. 3.—One of the ventral chete of an adult Arenicola. (Camera, Zeiss B.4.) x 150. 
Fie. 4.—Transverse section through middle region of the body, about Somite 16. 
(Camera, Zeiss D.2.) x 200. [The intestine has probably shifted a little from its natural 
position. The oblique muscles are not cut through in the section drawn, but I have added 
them from neighbouring sections.] Bl. ves. Blood-vessels passing to and from the gill 
(which is not represented in the section). chlor. Chloragogen cells around the ventral 
blood-trunk. circ. muse. Circular muscle of the body-wall. dors. v. Dorsal blood-trunk. 
epid. Epidermis. iné. Intestine. long. mus. Longitudinal muscles of the body-wall. 
n. cells. Nerve-cells. n. fi. Nerve-fibres. mu. circ. Nuclei of circular muscles. nu. long. 
Nuclei of longitudinal muscles. 007. mus. The oblique (or transverse) muscle. splanch. 
Nuclei of visceral ccelomic epithelium. vent. ves. Ventral blood-trunk. 
Fie. 5.—A section through a gill. ep. Epidermis, in which is a number of gland-cells 
(gi. e.). circ. mus. Circular muscles of the body-wall. Bl. ves. Branchial vessels. 
Fia. 6.—A portion of Fig. 1 more highly magnified. It represents one of the somites 
containing a nephridium. (Camera, Zeiss D. 2.) x 200. The body-wall and the intestine 
are represented by the shading. The chete of two neighbouring somites are included in 
order to show the relative position of the nephridial pore and funnel. a. The excretory 
region. 6. The “ duct.” 
Fia. 7.—A portion of the body-wall viewed from the surface; it shows one of the 
glandular bands (4) bounded by the narrow non-glandular rings (@, @) which occur in each 
somite. gl. Gland-cells. 
Fia. 8.—Diagrammatic transverse sections of the body to show vascular system in the 
branchial region on the right side, and the intestinal vessels on the left side. Br. ves. 
Branchial vessel. Dor. ves. The dorsal trunk. Vent. ves. Ventral trunk. Both are quite 
separate from the wall of the intestine (see Fig. 4). d. int. ves., v. int. ves. Vessels passing 
from these trunks to and from the plexus in the wall of the intestine (int. cap.). 
