galloway: non-sexual reproduction in dero vaga. 135 



4. The histological steps of bud formation in Dero agree for the most 

 part with those described for Chsetogaster by von Bock. This is true of 

 the orio-in of the brain from paired ectodermic ingrowths, the reinforce- 

 ment of the ventral nerve cord by serial ectodermic invasions between 

 the longitudinal muscle bauds, the ectodermic origin of the bristle sacs 

 in the newly formed segments, the origin of the pharynx from the ento- 

 dermal cells of the old gut, the paired ectodermic invaginations con- 

 cerned in the formation of the mouth, and the absence of a proctodseum 

 in the anterior individual. The chief points of difference which should 

 be noted are as follows. 



(a.) In Chsetogaster the old entoderm and the wall of the body cavity 

 unite directly in the anal segment. In Dero there is an outgrowth of 

 newly formed ciliated entoderm which unites the old entoderm with the 

 body wall. This new entoderm lines the pavilion. The anus is at the 

 place of union of the old and new entoderm. According to von Kennel 

 (82), there is a proctodeum in Ctenodrilus pardalis. 



(b.) While the mouth and pharynx of Dero and Chsetogaster present 

 certain common features in their mode of origin, there are important dif- 

 ferences between the two worms in each region. In Chsetogaster the paired 

 ectodermal invaginations ultimately run together across the median plane 

 to the mouth, and they secure union with the new pharynx before the 

 separation of the zodids. The ectodermic invaginations in Dero, on the 

 other hand, never meet, producing merely the buccal sinus at either 

 lateral angle of the mouth cavity. The floor and roof of the mouth are 

 formed by mechanical infolding of the appropriate portions of the anterior 

 margin of the body wall around the old gut. This old ectoderm is brought 

 into contact with the new entoderm and with the new ectoderm of the 

 invaginations mentioned above, after the separation of the zodids. The 

 pharynx is formed in Chsetogaster by an outgrowth of the entodermal cells 

 on the floor of the old gut. This cell mass comes to be occupied by a lumen, 

 single behind but continued forward as a pair of curved branches which 

 come to communicate with the ectodermal invaginations which are the 

 beginnings of the mouth. The old gut becomes constricted, and, together 

 with the nerve cord and the ventral body wall lying between the funda- 

 ments of the mouth, is resorbed. In Dero the pharynx is formed around 

 the old gut, especially well developed on the dorsal side, and the cavity of 

 the gut becomes the lumen of the pharynx, the old wall being split away 

 from the new entoderm, but not losing its continuity until the zodids 

 separate, whereupon the old wall is entirely broken down and lost. The 

 differences cited above are perhaps to be correlated in part with the fact 



