274 J. H. ASH WORTH. 



retractor muscle. Some such arrangement is necessary to 

 prevent injury to the delicate sense hairs when the animal is 

 burrowing" in the sand. 



The sense organs differ widely in structure according to 

 their age aud the size of the specimen from which they were 

 taken. Very young organs may be seen in sections of the 

 last few segments of an animal. 



Sense organs are clearly distinguishable in about the third 

 segment in front of the tail segment (fig. 27). The rods 

 are exceedingly small and difficult to see ; they occupy an 

 area equivalent to that of one or two epidermal cells. The 

 sensory area is only 10 n long. Below the rods are two or 

 three small ganglion cells about 8 — 10 ju long, and below 

 these are about twenty deeply staining nuclei. In the next 

 anterior segment the rods are rather more obvious, being 

 5 — 6 ju long, and in the segment further forward the sense 

 hairs are clearly visible, and have attained a length of about 

 5 — 6 /u. In these posterior segments the rods, ganglion cells, 

 and nuclei are closely compressed, and their relatious are 

 difficult to determine; but further forwards, as the sense 

 organs increase in size and the various structures become 

 better differentiated, their connections with each other may 

 be more readily seen. The foregoing description on pp. 272, 

 273, is taken from fully developed sense organs of specimens 

 13 — 15 mm. long. In older specimens there are still further 

 changes (fig. 30). The axial portion of the organ becomes 

 more fibrous, the ganglion cells undergo little change, but 

 there are very many more of the deeply staining nuclei at 

 the base of the organ than in younger specimens. The rods 

 also stain more homogeneously, their nuclei being almost 

 invisible, their position being indicated by a slightly darker 

 area in each rod. 



Occurrence of Lateral Sense Organs in other 

 Polychseta. 



Lateral sense' organs are proved to occur in only a very 

 few Polychaeta. They are found in the Capitellida^ (with 



