vm 



PHYLUM MOLLUSCOIDA 



337 



modified cells of the parenchyma, either of the funiculus or of the 

 body-wall. The testis, developed from the cells of the funicular 

 tissue, gives origin to spherical masses of cells the spermatidia 

 (sp) which develop into sperms with very long motile tails. These 

 become free from one another and move about in the body-cavity 

 or in its prolongations into the tentacles. There is no spermiduct, 

 and it is doubtful if the sperms pass to the exterior. The ovary 

 (ov) is a small rounded body formed from the parietal layer of the 

 parenchyma about the middle of the zocecium ; it consists of only 

 a small number of cells of which only one at a time becomes a 

 A p 



ect 



Fra. 282. Early stages in the development of Bugula. cent, central 

 corona ; ect. ectoderm ; end. endoderm ; seg. segmentation-cavity. 



mature ovum, certain smaller cells forming an enclosing follicle. 

 The mature ovum is perhaps fertilised in the ccelome ; iBpass 

 into the interior of a rounded outgrowth of the zooeciiSfc the 

 ooecium (OOBC.) lined with parenchyma, and forming a sort of m 

 pouch in which it undergoes development. \ " 



Development. Segmentation (Fig. 282) is complete am 

 nearly regular. A blastula is formed having the shape of a 

 bi-convex lens. In the interior of the blastoccele or cavity 

 of the blastula, four cells (end.) the primitive endoderm 

 cells become distinguishable ; these increase in number by> 

 division, and form a mass of free cells which almost completely 



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