]^30 John H. Gekould, 



may or may not be accompanied by the segmentation of the meso- 

 blastic bands and of the ventral nerve cord. 



The fact that the incipient metamerism in Phaseolosoma affects 

 chiefly the nerve cord, without any accompanying segmentation of 

 the somatic plate, is not easily reconciled with the idea which Heath 

 has expressed. Moreover the paired lateral bristles, which Selenka 

 found in Phaseolosoma, and which appear to be genuine signs of 

 segmentation, arise along the narrow lateral lines. The dorsal 

 ectoderm is entirely unsegmented. 



Comparison with the Echiurids. 



I agree with Hatschek that the Echiurids are Chaetopods. The 

 principal points at which they are differentiated from Sipunculids are: 



(1) The formation of the ventral nerve cord from two lateral 

 rudiments, 



(2) the character of the retractor muscles, 



(3) the terminal position of the anus, 



(4) the presence of anal vesicles, 



(5) the presence of a head kidney in the trochophore (in 

 Echiurus), 



(6) the presence of a prostomial proboscis, 



(7) the prominent segmentation of the trunk of the trocho- 

 phore and 



(8j the presence of several pairs of nephridia (three or four in 

 Thalassema). 



(1) The ventral nerve cord in Echiurids, as Hatschek (1881) 

 has shown, is established in two parallel rows of cell-groups, the 

 rudiments of ganglia. Thus two independent lateral cords are formed, 

 as in Chaetopods, and secondarily united by the differentiation of a 

 thin middle band from the ectoderm of the mid-ventral line. In 

 Sipunculids, on the other hand, the rudiment of the ventral nerve 

 cord consists of a single median band, as in Polygordius. 



(2) The ventral retractor muscles in the Echiurids, as described 

 by Hatschek and by Torrey (1903), resemble closely those of 

 Chaetopods. Thus in the trochophore of Eupomatus, as in Echiurids, 

 ventral longitudinal muscles occur w^hich consist of two parts, one 

 of which is preoral in its attachments, one postoral. The dorsal 

 and ventral retractor muscles of Sipunculids resemble the corresponding 

 muscles of the Molluscs and certain Chaetopods, but have no special 

 resemblance to those of the Echiurids. Indeed one would expect 



