358 Ralph S. Lillie 



of the limits of the intestinal regions, the main characteristies of 

 the adult body are complete. The activity of the growing zone 

 ceases when the larva has attaiued a length of from 2 to 2.5 mm., 

 at which stage the outward appearance and internai Organization 

 are essentially identical with those of the adult — further develop- 

 ment consisting chiefly in au increase in size and in the completion 

 of histological differentiation. In such larvae, the distribution of the 

 blood-vessels is substautially as in the adult, and the two "hearts" 

 connecting the gastric and subintestinal vessels at the anterior extre- 

 niity of the stomach, are already differentiated and rhythmically 

 contractile. The gills bave appeared in the 7*^ to 17*^ setigerous 

 somites (Vili — XVIII) as thin-walled evaginations of the dorso-lateral 

 body-wall containing looped blood-vessels. The otocysts are present at 

 the sides of the prostomium. The six nephridia are visible through 

 the transparent body-vrall as sac-like structures in the 5*^ to the 

 lo*'' setigerous somites (VI — XI). The dorsal setae and the ventral 

 rows of hooks bave the usuai charaeters, but bave increased greatly 

 in number as compared with earlier stages. Larvae of 10 — 15nim. 

 length bave undergone little further alteration, except in respect 

 to size and degree of histological differentiation. The gills bave 

 become more branched, the "hearts" more conspicuous and better 

 defined, the number of setae has increased, and the external body- 

 rings bave a})peared. At this stage in fact, the adult Organization 

 is essentially complete. 



4. Anatomy and histology of the adult nephridium. 



The six pairs of nephridia of the adult Arenicola cristata are 

 situated in the 5*^ to the IO*'' setigerous somites inclusive. These 

 somites correspond however, as seen from a comparison of early 

 larvai stages, to body-somites VI to XI, the first somite (peristomium) 

 being without setae, and in the adult, not sharply demarcated exter- 

 nally from the secoud. Internally, however, its posterior limit is 

 marked by the first diaphragm whose position corresponds closely 

 with that of the originai first sei)tum. Between the first and second 

 diaphragms intervenes a region composed of two somites; these are 

 in oarly stages perfectly distinct from one another; in adult life, 

 iiowever, the bisegmental composition of this region is less evident; 

 it is however, clearly indicated by the presence of two segmentai 

 tufts of notopodial setae, as well as by certain peculiarities of the 



