142 r. H. STEWAET. 



(2) The unilateral asymmetrical excretorj' organ, in its 

 anterior part flattened and band-shaped, and with a highly 

 modified niiclcns, as in A. decipiens (PI. 9, fig. 37). 



(3) A simihir organ, but without the band-like enlargement, 

 as in A. clavata (PI. 9, fig. 38). 



(4) The bilateral organ of, e. g., A. megalocephala 

 (PI. 9, fig. 40). 



In all these types the organ consists of a single lai'ge cell, 

 with an intra-cellular system of canals, and with a duct formed 

 in many cases by an invagination of epidermis. All four are 

 homologous, an intermediate type between (2) and (4) being 

 found in A. rotundata, in wliich a small limb crosses from 

 the main stem of the gland on the left side to the right 

 (PI. 9, fig. 39). The cause of the change in type from (1) 

 through (2) and (3) to (4) is to be found in an increase of 

 work thrown on the gland, probably by change of habits in 

 the animal. The gland is compelled to enlarge, and adapts 

 itself to the narrow body-form by elongating, and following 

 the line of least resistance, applies itself first to one and then 

 to both lateral lines. 



Bastian (1) had previously (and he is quoted by Jiigerskiold) 

 stated the same opinion, that the ventral gland of free Nema- 

 todes was homologous with the excretory organ of parasitic 

 forms. 



The results given above supply another link in the chain. 

 The excretory gland in the embryo above described (PI. 9, 

 fig. 36) is a typical ventral gland, inasmuch as its opening, 

 although situated close to the mouth, is also immediately in 

 front of the nerve-ring (corresponding Avith the situation in 

 Oncholaimus vulgaris), and as it lies free in the body 

 space, comes into contact only with the left lateral line, never 

 being in continuity with it. 



On the other hand, in many points it resembles the excretory 

 oi-gan of A. decipiens. Its duct is formed by an ingrowth 

 of cells from the ventral line ; its anterior portion is broad, 

 almost band-like, and contains the nucleus ; its posterior por- 

 tion is narrow and thread-like. It contains a central canal 



