METAZOA VERMES. 293 



pre-oral lobe in front of the mouth decreases in size (fig. 

 4) . This process goes on until the form of the mature 

 larva (fig. 5) is attained. The body is now made up of 

 similar segments ; the mouth is on the ventral side, and 

 there is an ocellus on either side near the forward end, 

 while it possesses (in addition to the young larva) a pair 

 of short tentacles (fig. 5). These are its only appendages, 

 and as there are no locomotive orgarrs in the form of 

 chitinous setae or fleshy paddles, the animal moves by 

 means of cilia only. Neither does this worm possess 

 external breathing organs in any form, for the skin proba- 

 bly performs the work of respiration. 



The adult (PI. 728, fig. i, natural size ; fig. 2, enlarged) 

 does not differ essentially from the mature larva. Some 

 of its features enlarged are shown in figs. 3-5. The body 

 is segmented, the segments showing more plainly in the 

 enlarged section (fig. 3) than in figs, i, 2. The pre-oral 

 lobe bears the pair of hairy tentacles (fig. 4) and back of 

 this lobe is the mouth (fig. 4). The anus is still at the 

 posterior end of the body (fig. 5) surrounded by a circle 

 of papillae. 



The worms which are best known and which are exhib- 

 ited in the Synoptic Collection belong for the most part 

 to one of two groups. Either they are broadly differenti- 

 ated by the possession of many parts and organs, or they 

 are extremely specialized by the reduction of organs. 

 The first group to be described consists mainly of marine 

 and shallow-water forms, and many are found for a longer 

 or shorter time under stones or burrowing in sand. They 

 have become more or less adapted to their environment, 

 so that some of their organs are secondarily acquired and 

 these of course throw little light on phylogenetic relation- 

 ships. The second group, however, consists mostly of 

 parasites, and their structure has undergone such a pro- 

 found modification that they are much farther removed 

 from the primitive ancestral forms than are the members 

 of the first group. They have become adapted to the 



