Post- Larval Stages of Arenicola 79 



arranged as in the adult, and six pairs of tubular nephridia * are 

 present. 



Post-larval examples of A. marina are from 3'5 to 8'5 mm. 

 long, and may be identified by reference to their general form, their 

 nineteen chaetiferous segments, the presence of six pairs of nephridia 

 (the pores on the fourth to the ninth segments are generally visible), 

 a single pair of oesophageal glands, and open statocysts. 



Post-Larval Stages of ARENICOLA CRISTATA. 



Near Beaufort, North Carolina, Prof. E. A. Andrews 2 found, 

 enveloped in a gelatinous tube, a young Arenicola, evidently 

 belonging to the species A. cristata, in which statocysts were 

 present, each containing a single large statolith. There is no other 

 record of the capture of a post-larva of this species, but Dr. E. S. 

 Lillie has reared specimens in aquaria. 



The full number of body-segments, that is, seventeen chaetiferous 

 segments, is acquired by the time the young worm is about 2 mm. in 

 length. When it has attained a length of 5 mm. about twenty tail- 

 segments have also been formed, and gills are beginning to make 

 their appearance on the posterior chaetiferous segments. The worm 

 continues to grow in length, by elongation of the body- segments and 

 by the formation of new tail-segments, until the latter have reached 

 a definite number, namely, thirty-eight to forty (PL X, Fig. 30). 

 Meanwhile, the formation of gills proceeds from behind forwards 

 until these organs appear on the seventh segment. The end of the 

 post-larval stage has then been reached, and the worms have become 

 young adults in structure and in form, and are about 6 to 7 mm. long. 

 Most later phases present a tail region comprising considerably fewer 

 than forty segments, due to the readiness with which segments are 

 lost from the posterior end. 



The principal diagnostic features of post-larval stages of this 

 species are seventeen chaetiferous segments, six pairs of nephridia 

 (their pores on the fifth to the tenth segments), one pair of 

 oesophageal glands, a pair of moderately large septal pouches, and a 

 pair of closed statocysts, each containing a single statolith. These 

 characters can be seen in most preserved examples, which have been 

 stained lightly, and cleared carefully in cedar-wood oil. 



1 For an account of their structure, see the writer's Liverpool Mar. Biol. 

 Comin. Memoir (1904), p. 67. 



2 Proc. U.S. Nat. Museum 1891, xiv (1892), p. 300. 



