Development of Arenicola 75 



a feeding organ, and food particles are passed into the oesophagus. 

 The subsequent stages of larval development have been followed 

 only in A. cristata, to which species, therefore, the succeeding part 

 of this account relates. New segments are formed in the growing 

 zone immediately in front of the anal segment (pygidium) until the 

 full number of chaetiferous segments is attained. Henceforward the 

 segments formed (thirty-eight to forty in number) are of a different 

 nature and are without chaetae (PI. X, Fig. 30). By the time that 

 about twenty tail-segments have been formed, gills begin to make 

 their appearance on the posterior chaetiferous segments in the 

 manner described on p. 55, The changes which take place in 

 the internal organs have been traced only in relation to the septa, 

 nephridia and alimentary canal. Septa are formed and are for 

 some time present between all the chaetiferous segments, and pro- 

 nephridia are developed in association with the third to the tenth. 

 The first two of these pro-nephridia become degenerate ; the others 

 are transformed into the nephridia of the post-larva. In the 

 meantime the regions of the alimentary canal have become more 

 clearly marked; blood, blood-vessels and hearts have been formed, 

 and the nervous system and sense-organs have become more fully 

 differentiated. 



POST- LARVAL STAGES, WITH A DISCUSSION OF THE GENUS 

 OLYMENIDES. 



Prof. Benham applied the term " post-larval " to that stage of 

 development of Arenicola marina in which the worm has attained 

 the full adult number of chaetiferous segments, and is divisible into 

 an anterior chaetiferous region and a posterior achaetous region or 

 tail, but in which the gills are not yet completely formed or have 

 not even made their appearance. 



Post-larval stages of five species of Arenicola are known, namely, 

 A. marina, ecaudata, branchialis, assimilis and cristata. Those of 

 the two first-named species have been the subject of considerable 

 discussion, because by some writers they were regarded, not as 

 stages in the growth of Arenicola, but as belonging to a separate 

 genus, Clymenides. 



There can be little doubt that the worm described by Claparede 

 in 1863, 1 and referred by him to a new genus and species 



1 Beobacht. Anat. wirbell. Thiere Normandie (1863), p. 30, taf xv 

 figs. 24-27. 



