1890.] Annelid Descent. 1147 
I imagine the ancestors of Annelids to have been powerful, 
predatory Turbellarians, which lived a pelagic life, and were at 
that time masters of the seas. By their agility in swimming and 
in catching their prey these forms were distinguished from their 
present non-parasitic relatives, the Planarians; and only such ac- 
quirements could, in my opinion, have led to a perfection of 
organization in an ascending direction, The Planarians were 
forced to the sea bottom to take refuge amongst rocks and plants, 
first by these ancestors, then by their favored younger relatives, 
the Annelids, and finally by fish and other predatory marine 
animals. Here they led a creeping life, and thence gradually 
acquired a flat, broad body, with irregular arrangement of the in- 
ternal organs. The ancestors had an elongated body, rounded in 
section and very supple; so that these may have borne a resemblance 
to the Nemerteans. Yet the Annelids cannot be derived from 
these worms, since they undoubtedly form a side branch, subse- 
quently much changed and distinguished by peculiar characters 
(proboscis, vascular system, excretory organs) Yet these have 
preserved some of the internal organization derived from those 
common pelagic ancestors, which may give us very valuable hints 
in considering the Annelid body. 
In the body parenchyma, partly surrounded by, partly pene- 
trated by powerful muscle systems, were the sexual glands, 
appearing originally in the immature state as a single pair of 
compact cell-bands, but in the mature condition as long, hollow 
tubes, opening externally at the posterior end of the body by a 
pair of simple dermal pores. It is conceivable that these organs, 
being distended with eggs or sperm at certain times, would much 
interfere with the flexibility of the entire body. Now, however, 
in consequence of this very rigidity produced by the excessive 
accumulation of sexual products, they would finally yield to the 
constantly repeated endeavors of the animal to regain its accus- 
tomed mobility, and divide up into smaller glands. Thus it would 
have been the serpentine swimming motions (by which alone we 
can imagine the rapid change of place of a long worm in the 
water) of the Turbellarian-like ancestors of the Annelids which 
caused the disintegration of the two originally uniform, elonga- 
