THE BEISTLB-WOBMS. 105 



the palps arise from the ventral surface, the tentacles are 

 dorsal. Perhaps one of the most interesting things about 

 this, as about most of the other species of Nereis, is the 

 changes which it undergoes at sexual maturity. When hunt- 

 ing under stones for specimens you may not infrequently 

 find one which is peculiar in that while the anterior part of 

 the body has all the usual characters, the posterior region is 

 strikingly different in appearance, so that the worm looks as 

 though it were compounded of two dissimilar worms. The 

 colour is also remarkable, for the body is bright green 

 anteriorly, and pure pink in the posterior region. If you 

 examine the posterior region more closely, you will find that 

 the difference in appearance is largely due to the modifica- 

 tion of the parapodia. These have greatly increased in size, 

 and their different regions have developed leafy outgrowths, 

 which convert the parapodia as a whole into swimming 

 organs. The resultant change in the external appearance of 

 the animal is so striking that the modified form was for long 

 supposed to belong to a distinct genus, and was called 

 Heteronereis. It is now known to be merely the mature 

 form, and owing to the fact that it is adapted for a free- 

 swimming existence no doubt assists in the distribution of 

 the species. It will be remembered that in the Coelentera 

 the sessile sea-firs bud off active swimming- bells, which 

 produce the ova, and by their power of swimming ensure 

 the distribution of the species. In certain small worms 

 belonging to the genus Syllis and to allied genera something 

 quite analogous occurs, for the worms bud off new indi- 

 viduals, which are modified for a free-swimming life, and 

 which contain the eggs and spermatozoa. In another very 

 curious worm, the "Palolo" of Samoa and Fiji, a portion of 

 the body becomes modified, much in the same way as in 

 Nereis, but the modified portion separates off, and swims 

 away, leaving, it would seem, the anterior region behind at 

 the bottom. The separated portions of the worm appear at 

 the surface of the water in extraordinary numbers at certain 

 seasons, and are caught and eaten by the natives. The 

 " swarming " only lasts for a short time, and it is probable 

 that the worms die after laying their eggs, while the 

 "heads," which have remained at the bottom among the 

 coral blocks, bud out new bodies, and eventually repeat the 



