THE SOMITliS. 139 



and probably coustitutes an important functional part of the 

 salivary gland. 



Somites of the first, second, and third Legs (Part III, p. 

 90 et seq.). — The fourth, fifth, and sixth somites divide early 

 into a dorsal and ventral portion, of which the dorsal vanishes, 

 while the ventral acquires an opening to the exterior and persists 

 as the nephridiura. The condition of these nephridia in Stage f 

 is shown by PI. VIII, fig. 40. The subsequent changes are 

 very slight, and may be gathered from an inspection of 

 PI. XI, fig. 10, which is from a transverse section through 

 the third leg of an embryo almost ready for birth. The 

 nephridium (Pi. XI, fig. 10) consists of a thin- walled internal 

 vesicle contained in the leg compartment of the body cavity 

 and communicating by a straight tube with the external 

 opening on the ventral surface. The wall of the vesicle 

 consists of a ragged protoplasmic layer, with here and there a 

 round nucleus. 



Somites of Legs 4 to 12. — The early history of the seventh 

 to the fifteenth somites inclusive is similar to that of the somites 

 of the first three legs ; but in the later stages the tubular part 

 of the nephridium becomes elongated, coiled, and divided into 

 at least three regions (PI. XIII, fig. 17, diagram). (1) The 

 part next the external opening is dilated into a vesicle — the 

 external vesicle — which is connected with the external opening 

 by a narrow tube (PL XI, fig. 11). (2) The vesicle opens 

 into a long coiled tube, which forms the greater part of the 

 nephridium. It is cut across twice in the transverse section 

 from which fig. 11 was taken. It is continuous with (3) a 

 short terminal portion in which the nuclei are very closely 

 packed together. This terminal portion opens with everted 

 lips into the thin-walled, internal vesicle, and constitutes the 

 so-called funnel of the nephridium. The external opening 

 of the nephridia of the fourth and fifth legs are at first imme- 

 diately outside the nerve cord, as in the case of the others. 

 Their adult position is due to a secondary shifting. 



Somites of Legs 13 to 17 (p. 93). — The dorsal divisions 

 of these somites persist as the generative organs, and will 



