vii ANNELIDA 165 



tions of cells from the intact ectoderm. In this way he accounts for 

 that characteristic feature of development, the casting off of the 

 prototroch and the union of the head and trunk blastema. The 

 segmentation of the mesoderm and coelom he brings into connection 

 with the wriggling method of progression which replaced ciliary gliding. 



It is worthy of note that on this hypothesis, segmentation can only 

 have appeared after the burrowing life was assumed, and the appear- 

 ance of segments in the free-swimming larva, which is seen even in 

 Polygordius, must be regarded as a " telescoping " of development in 

 comparison with what actually occurred in the history of the race. 

 To similar conclusions we are driven by the study of almost all 

 classes of larvae, so there is nothing unlikely in Woltereck's theory, 

 which we may indeed provisionally adopt as by far the best solution 

 of the problem of the origin of the Annelida which has as yet been 

 offered. 



We have, however, another problem to face. There appears in 

 the Annelida for the first time, an organ widely distributed in the 

 animal kingdom, over whose nature and origin many battles have 

 been fought. We refer to the coelom, often aptly termed by the 

 German authors the secondary body-cavity in order to distinguish 

 it from the space which first appears in the embryo termed the 

 segmentation cavity or blastocoele ; the segmentation cavity is 

 termed by the Germans the primary body-cavity, and it forms 

 the space intervening between skin and gut in the Trochophore. 



Now the investigations of the American authors show, that the cells 

 forming the wall of the coelom always originate by the division of a 

 single cell 4d, whose sisters 4a, 4b, and 4c, enter into the formation 

 of the endodermic wall of the gut, while 4d itself contributes cells to 

 the gut wall. It is, therefore, reasonable to suppose that 4d itself at 

 one time formed part of the endoderm, and that the adult mesoderm 

 is of endodermic origin. The final demonstration of this belongs to 

 Shearer (1911), who has shown that in Uupoviatus 4d actually forms 

 part of the endodermic wall of the larva for a whole day after the 

 free-swimming life has begun. 



But what changes in successive generations of adults can we 

 suppose to be represented by the separation of a gut -cell from its 

 neighbours, and by its proliferation to form a mass of cells which 

 later become hollowed out to form a cavity ? By far the easiest and 

 most natural suggestion is that the process in the larva represents 

 the separation of a pair of endodermal pouches or of a single bilobed 

 pouch from the gut, which eventually became completely shut off 

 from the main gut and devoted to other uses. This interpretation is 

 borne out in the strongest manner by the actual origin of the coelom 

 in other groups where it occurs, and when we are treating of the 

 Echinodermata the question will be discussed more fully. 



The alternative theory which was originally put forward by 

 Meyer (1887) is, that the separation of the mesodermic cells from the 

 gut wall represents an outward migration of the primitive genital 



