ANNELIDA 



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The body cavity of the trunk in such cases therefore does not 

 represent the cavity between the two layers of the mesoderm, but corre- 

 sponds to the blastoccele (traversed by mesodermal cells), just as the 

 cephalic cavity does in the case mentioned above. Moreover, in Lopad- 

 orhyuchus this also arises by an immigration of mesoderm cells into the 

 head portion. From this description it follows that the formation of the 

 body cavity does not always take place in so regular a manner as has 

 been described above ; in fact, according to Kleinenberg's statements, 

 the formation of the entire mesoderm may be effected in another manner. 

 It has already been mentioned (p. 286) that in Kleinenberg's opinion 

 the ectoderm, in addition to the pole cells, takes part in the formation of 

 the mesodermal bands of Lumhricus trapezoides. Cells are separated off 

 from the ectoderm and added to the germ bands lying under them. In 



Pig. 135— Sagittal section of a larva of Lopadorhynchus (after Kleinenbebg)/ 

 d, intestine ; mp, muscle-plate ; np, neural plate ; ces, fundament of the (permanent) 

 oesophagus ; sg, fundament of the supra-oesophageal ganglion ; so, apical organ ; 

 st, stomodseum (temporary fore-gut of the larva) ; w, preoral band of cilia. 



Lopadorhynchus Kleinenbero derives the entire mesoderm from the 

 ectoderm. According to him, mesoderm does not exist as a separate 

 layer. The musculature of the Lopadorhynchus larva arises by means 

 of an emigration of cells from the ectoderm (Fig. 134 A to C). The so- 

 called muscle-plates (Fig. 135 mp) are formed by a splitting of the 

 thickened ventral ectoderm, first at the hind end of the larva and then 

 successively further forwards. The course of this cell growth, leading to 

 the formation of the muscle-plates, is evident from the figures 134 A to C. 

 The muscle-plates of the two sides are separated by a fold of the ento- 

 derm. The segmentation of the muscle-plates takes place after the 

 fundaments of the setigerous sacs have grown in from the neural plates 

 (comp. p. 287 and Fig. 134 C). The limits of the segments arise by the 



