ANATOMY AND CLASSIFICATION OF THE ARENICOLID.Tl. 455 



mature A. ecaudata the ovaries or testes are so voluminous 

 as to conceal the greater part of the nephridia and the 

 alimentary canal. There are the usual three anterior dia- 

 phragms placed exactly as in A. marina, the first being 

 oblique and inserted dorsally, one annulus in front of the first 

 notopodiura, ventrally just behind the first ueuropodium ; 

 while the second and third are placed vertically, two annuli 

 behind the second and third parapodia respectively. From 

 this point the coelom is uninterrupted by septa for a con- 

 siderable distance. The septa, in fact, are reduced to 

 strands of connective tissue binding together the afferent 

 and efferent vessels which are connected with the nephridia, 

 and in the '^ marina " section with the anterior gills. This 

 tissue increases in amount so as to form a complete septum 

 opposite the last gill of A. Claparedii and A. crista t a, and 

 regularly between the caudal segments in these forms (figs. 

 26, 30). In A. ecaudata and A. Grubii the complete 

 septa commence at the beginning of the branchial region, and 

 are continued to the end of the body (PI. 25, figs. 44, 45). In 

 these species also the tissue supporting each of the vessels 

 going to the nephridia from the ventral vessel may be 

 regarded as an incomplete septum, since it is expanded peri- 

 pherally, and inserted partly into the nephridium and partly 

 into the body-wall in the region of the second groove behind 

 the chgetigerous annulus — the usual limit of the segments. 



The first diaphragm in A. cristata, A. ecaudata, and A. 

 Grubii is stout, and produced backwards into a pair of long, 

 finger-shaped, contractile vascular sacs, 5 to 12 mm. long, 

 which lie at the sides of and slightly ventral to the oesopha- 

 gus. These diaphragmatic pouches are entirely absent in A. 

 Claparedii. On opening specimens which are sexually mature 

 a mass of spermatozoa or ova is often found in connection 

 with the third diaphragm. The genital products accumulate 

 either in front of, or more usually belund the diaphragm, and 

 cause it to become distended, forming a pouch, or often two 

 pouches or sacs, one on each side of the alimentary canal. 



In A. cristata there is, in the tail, a well-marked dorsal 



