352 Ralph S. Lillie 



is as distinct as in the ndult. The posterior achaetous region under- 

 g-oes relatively little further dilìerentiation, and its somites exhibit 

 throughont lite an alraost complete resemblanee to one another. The 

 addition of new somites continues in the usuai manner uutil the 

 definitive number has been attained, after which tlie growing zone 

 loses its distinctive characters and becomes indistingnishable as such. 



The disappearance of the growing zone and the cessation of 

 somite-forraation take place with great regularity at a stage of 

 about 58 somites. The final number of achaetous somites is thus 

 found to be approximately Constant; it may be accurately determined 

 by counting the septa in longitudiual sections of specimens in which 

 the growing zone has disappeared. In such larvae the number is 

 found to range from 38 to 40, and appears never to exceed or fall 

 below this. The total number of somites formed in this species of 

 Arenicoki seems thus approximately Constant (from 56 to 58); in adult 

 specimens, however, the number actually found is generally con- 

 spicuously sraaller than the above; the detìciency is readily accoun- 

 ted for as due to a loss of a portion of the posterior region by 

 autotomy, which is of frequent occurrence in this section of the body. 

 Although the posterior regiun increases in absolute size with 

 the growth of the animai, it undergoes little further diflerentiation, 

 and reraains throughont life in a primitive uniformly scgmented and 

 achaetous condition. The presence of both dorsal and ventral 

 mesenteries, the regulär repetition from end to end of complete 

 septa, the simple uniform structure of the intestine and the regulär 

 metamerie repetition of the circura-intestinal blood-vessels, are other 

 characteristic features of this region ali of them of a decidedly 

 primitive kind. Ali of these features appear early in the deve- 

 lopment of the region, and persist throughont life. 



With the anterior chaetigerous region the case is very different, 

 and very extensive further alterations are necessary before the defi- 

 nitive structure is finally attained. Fig. 30, Piate 23 represents a 

 sagittal section of a larva at a stage of about 8 somites. The 

 metamerism, it will be observed, is expressed in a relatively 

 simple manner: the ventral eetoderm is thiekened at segmental inter- 

 vals, and the septa, ali of which are incomplete dorsally (except 

 those just forming), extend from a region a little in front of the 

 constriction ujìwards and oblicjuely backwards to the sub-intestinal 

 blood-vessel. The first septum is inserted into the Oesophagus 

 immediately behind the proboscis, and into the body-wall almost at 



