532 Causes and Course of Organic Evolution 



In Synchceta pectinata four pairs of setigerous appendages 

 can be traced. '*At the summit of the dorsal hump meet 

 two rocket-headed antennae, each bearing a tuft of setae; and 

 the two tufts issue together from the same opening. A nerve 

 thread passes from each rocket head to a nervous ganghon 

 in the head" (169, I: 125). In front of these are two latero- 

 ventral antennae, each being a small tube that bears a swelling 

 into which one or more styles are set. A third pair, corre- 

 sponding to the buccal pair in Asplanchna probably, is placed 

 more ventrally. But further, slightly behind the level of the 

 mouth, and on either side of the front body region, are two 

 "tongue-shaped fleshy protuberances, edged with powerful 

 cilia, and as they can be set by special muscles at various angles 

 to the body, the creature can dart, wheel, and stop, with the 

 greatest ease." In Polyarthra four pairs of appendages seem 

 also to be represented. 



The above comparative details show that gradual evolution 

 of two to four pairs of setigerous appendages may arise, and 

 that even in the same genus each pair may be distinct, or may 

 fuse into a median dorsal or sub-ventral appendage. All 

 accounts agree in stating also that special muscles pass to and 

 cause rapid motion of these, when they reach a fair size. Nerves 

 also are as exactly distributed to them as to the eyes. And, as 

 in Philodina, each may be simple or two to three-jointed. 



Such conditions lead us by stages to the unique genus Pe- 

 dalion, in which are a small median dorsal antenna, that prob- 

 ably represents a fused pair, and two latero-ventrals below 

 and on either side of the ciliated disc. Behind these are two 

 pairs of gigantic lateral setigerous appendages, a very large 

 median ventral one that may result from paired fusion, and 

 a like though smaller dorsal one. In this case then six pairs 

 of setigerous appendages are suggested, four pairs of which 

 are clearly expansions of the body substance, and are moved 

 by powerful muscles. In addition a pair of elongated seti- 

 gerous appendages grows backward in telson-like fashion from 

 near the anus. 



The two pairs present in most non-loricate rotifers, as dorsal 

 or lateral processes, may be correlated with the first and second 

 pair of antennae in Crustacea, the supra-oral pair seen in 

 Asplanchna as the mandibles, and the slightly more postero- 

 ventrals as a pair of maxillae. Possibly some of the large 

 appendages in Pedalion may represent the first and second 

 maxillae, and others the ambulatory appendages. The two 

 anal appendages seem entirely to agree with the furcal pro- 

 cesses of the Crustacean larva. All of the above is in complete 



