404 Ralph S. Lillie 



fibres of the bodj'-wall. Tliese represent the first appearance of the 

 iiiusculature of the terminal vesicle (compare Figs. 45, 53, 55). X 800. 

 Fig. 4 1 . Horizontal section through ventral region of somites VII and Vili of a more 

 advanced larva (ca. 3 nini. ; full nuniber of somites), showing the origin 

 of the segmentai vessels from the ventral blood-vessel , and the rela- 

 tions of one of the vessels to the 2nci nephridium of the right side. 

 X200. 



Piate 25. Arenicola cristata. 



Fig. 42. Longitudinal section through the left nephridium of somite VII (2nd de- 

 finitive nephridium) and adjacent body-wall of the larva of Fig. 41, 

 showing (1) a portion of the nephridial blood-vessel to which is 

 applied a large germ-nucleus; (2) the posterior junction of the nephridium 

 with the ectoderm (corresponding to the early undifferentiated terminal 

 vesicle); and (3) the cubical cells of the early nephrostome with their 

 shorter cilia. The lumen has extended throughout the entire length 

 of the organ and seems to communicate with the exterior; the external 

 opening, however, is very minute. X 800. 



Fig. 43. Cross section through the anterior end of the first left definitive 

 nephridium (somite VI) of a larva of about the same stage as Fig. 42, 

 showing the early condition of the septal blood-vessels of this somite, 

 and their relations to the nephridia and to the body-wall. Portions 

 of the primitive septum stili remain. X 800. 



Fig. 44. Longitudinal section through the anterior part of the A^^ nephridium 

 (somite IX) of a larva of a somewhat more advanced stage than Fig. 43 

 (length ca. 3 mm.). The nephrostome is turned somewhat inward and 

 has been cut in such a way as to show its epithelium in cross section; 

 the figure shows also its roLitions to the nephrostomial blood-vessel, 

 which runs across its dorsal lip and is in dose relation to the trans- 

 verse band of muscle [t. m. f.), which is also attached to this lip and 

 binda it down to the ventral body-wall. A portion of the primitive 

 septum in which the blood-vessel appears is seen at s. X 800. 



Fig. 45. Cross section through the posterior end of the l^t nephridium (somite 

 VI) of a larva in which the terminal vesicle is beginning to differen- 

 tiate (length ca. 3.5 mm; full no. of somites). The terminal portion 

 adjoining the ectoderm is thin walled and somewhat dilated. A band 

 of muscle-fibre (derived from the longitudinal layer of the body-wall) 

 extends across the terminal vesicle at its junction with the glandulär 

 region proper. Granules in the ectoderm near the terminal vesicle. 

 X 800. 



Figs. 46-50. These figures represent successive transverse sections, 7.5 \i. thick, 

 through the nephrostome and anterior glandulär portion of the first 

 nephridium (somite VI) of a larva of ca. 4 mm. in length. The nephro- 

 stome has beconie well-defined and is composed of a single layer of 

 small cubical ciliated cells; along its dorsal lip runs the nephrostomial 

 blood-vessel. The continuity of this vessel with the nephridial vessel 

 is shown. The relation of the segmentai blood-vessel to the trans- 

 verse muscle-band attached to the dorsal lip is also shown. /, 

 vacuolated wandering cells (excretophores?). X 800. 



