298 SYNOPTIC COLLECTION. 



quence of this early condition, it may be urged that here 

 is a proof of the descent of the segmented Annelids from 

 the group of flat, unsegmented worms. 



We must not lose sight of the fact, however, that this 

 unsegmented larval state is accompanied by a specialized 

 mode of reproduction, as already shown. The formation 

 of a pouch for the young, the brooding by the mother, 

 the various changes in the development of the asexual and 

 sexual individuals, all indicate secondary instead of pri- 

 mary conditions, so that it seems most probable that this 

 flattened state of the young larva is an adaptive character. 



The proboscis which we have seen in Nereis is some- 

 times of great size, as in Rhynchobolus siphonostoma (No. 

 738), a large, distinctly segmented worm. In this finely 

 preserved specimen the proboscis is extended and the 

 four dark horny teeth are seen at the swollen anterior end. 

 One row of tiny parapodia with setae extending from them 

 is seen on either side of the body. 



Some of the worms of this group of Chaetopods have 

 the head so slightly differentiated that the two ends of the 

 body resemble each other, as in Eteone siphonodonta (No. 

 739). They are tapering and terminate in a blunt point; 

 the forward end, however, differs from the posterior in 

 having short tentacles. 



The locomotive organs in Eteone are prominent, con- 

 sisting of a double row on each side of the body. The 

 upper and longer organs end in flattened leaf-like parts, 

 while the lower have clusters of setae. These enable the 

 worm to move swiftly through moist sand and also to 

 penetrate clay and fissures in rocks. 



Another worm with an inconspicuous head is Halla 

 parthenopeja (No. 740), which is of great size with a large 

 number of apparent segments. Florence Buchanan 1 has 

 shown that in a few Annelida (Chaetopods) cases occur 

 of intercalation of half segments and of spiral segmenta- 



1 Quart. Journ. Micr. Sci., XXXIV, 1893. 



