62 Arenicolidae 



musculature has become associated with it. In all the species, 

 except A. pusilla and assimilis, this septum bears two backwardly 

 directed muscular pouches which lie to the sides of, and ventral to, 

 the oesophagus (PL XIII, Fig. 46), and open into the coelomic space 

 in front of the septum. The function of the pouches is unknown, but 

 it has been suggested that they may be of use in aiding the eversion of 

 the proboscis. The relative size of these pouches is practically constant 

 in the members of the same species, but varies considerably in the 

 different species ; the grade of development of these structures, 

 therefore, forms a useful systematic character. Septal pouches are 

 not represented, even as vestiges, in A. imsilla and assimilis; in 

 A. marina and ylacialis they are small, conical or thumb-shaped 

 structures, only about 2 to 3 mm. long in full grown specimens. 

 The pouches of A. ecaudata and hranchialis are finger-like, extend 

 backwards almost as far as the second septum, and, in large speci- 

 mens, are 5 to 8 mm. long. Those of A. cristata are usually of similar 

 shape and length, but in very large American specimens they attain 

 a length of 13 mm. The septal pouches reach the highest grade of 

 development in A. loveni, in large examples of which they are 

 25 to 26 nun. long and about 3 to 4 mm. in diameter at their widest 

 part (PI. IV, Fig. 11). They extend backwards through apertures in 

 the second septum, so that their blind ends lie against, or almost 

 touching, the third septum. The enormous size of the septal pouches 

 is the most striking feature in a dissection of this species. 



The second and third septa are more extensive than the first but 

 much thinner ; they are fenestrated and permit the passage of the 

 coelomic fluid and corpuscles. 



From the third septum, that is, the anterior end of the fourth 

 segment, backwards to about the fifteenth segment the body-cavity 

 is uninterrupted by septa, but about the latter region vestigial 

 septa may be recognised as strands of connective tissue of greater or 

 less width accompanying some of the afferent and efferent branchial 

 vessels. In the succeeding segments the septa are more perfect, 

 and in the nineteenth segment, if not earlier, form easily recognisable, 

 and, at first sight, complete partitions. These septa, which are not 

 usually fenestrated, are, however, incomplete ventrally, that is, above 

 the nerve-cord, and possibly also mid-dorsally. They are well seen 

 in A. ecaudata and A. hranchialis, and are present, though demon- 

 strable with some difficulty, in the tail region of the caudate species, 

 where they correspond in position with the inter-segmental grooves 

 present on the external surface. 



