122 Arenicolidae 



at the anterior end ; the tail is generally yellow — chrome to gamboge — 

 and the gills light red. 



Large specimens are reddish brown in the middle region, their 

 anterior end is often yellowish or darkened, the tail is yellow or 

 greenish yellow, and the gills reddish brown. 



The American specimens had apparently been, in life, darker in 

 colour ; some are brown (in alcohol or formalin), but others, especially 

 those from California, are very dark — almost black, and similar in 

 colour to preserved specimens of the dark variety of A. marina 

 {cf. PI. I. Fig. 3). 



Vakiations in the Organs. — The varied forms assumed by the 

 prostomium may be seen on reference to Figs. 51-54, 48, 49). 



In examples from the coast of California there is a strong 

 tendency to tlie reduction and loss of the first gill. Out of seven 

 specimens from that coast only two have the full number of gills, and 

 in one of these the right and left first gills are very small ; specimens 

 from otlier localities possess practically constantly the full number 

 of gills. 



There is a clear, and sometimes a striking, difference in the 

 number of oesophageal glands exhibited by Mediterranean and 

 Pacific specimens. The former seldom have more than four pairs, 

 or at the most five pairs of caeca (PI. VIII, Fig. 18), whereas the 

 writer has seen only one American example with as few as five 

 pairs, others had six to ten pairs (PL XIII, Fig. 44), and Dr. Johnson 

 records specimens with fifteen and sixteen pairs. 



Period of Maturity, Development. — Lo Bianco (1909) stated that 

 A. jmsilla is sexually mature, in the Bay of Naples, from November 

 to May. The writer found that artificial fertilisation was successful 

 from April lyth to IMay 16th, 1900, and that worms collected after 

 the latter date had shed their genital products. 



Nothing is known of the form in which the ova are deposited. 

 The egg-cleavage and young larvae (p. 74) have been described by 

 the writer, l)ut later larval stages and post-larvae are unknown. It 

 is remarkaljle that, in spite of the daily tow-nettings taken in the 

 Bay of Naples, and the careful examination to which the plankton is 

 subjected in the Zoological Station, the post-larval stages of ^. imsilla 

 have not been met with. Possibly the habitat of these stages is 

 different from that of the corresponding phases of A. marina and 

 A. assimilis, and they do not come into the surface waters. 



