INTRODUCTION. XC1 



in a zocecium containing ova (in Membranipora membra- 

 nacea), I now consider it to be an excretory organ, which 

 serves at the swarming-season as the passage by which the 

 spermatozoa escape from the cell into the surrounding 

 water (= the nephridium, Ray-Lankester) . 



The ovary, as we have seen, varies in position ; it also 

 varies remarkably in size. In some species it contains a 

 large number of ova (thirty in Hypophorella, and pro- 

 bably an equal number in Farrella), in others only one 

 or two. Frequently two ova are produced, which are 

 either matured in succession, or one of them perfects its 

 development at the expense of the other, which is atro- 

 phied. It appears that in some cases, after the first ovary 

 (on the funiculus) has discharged its contents, it is fol- 

 lowed by a second. In its early stage the funicular ovary 

 is a minutely granular body, of an oval form, and a pale 

 golden colour (Woodcut, fig. xxiv. ov). In its interior 

 the ova make their appearance, and, when mature, consist 

 of a granular yolk (vitellus), which is sometimes coloured, 

 invested by a membrane, and presenting a conspicuous 

 germinal vesicle, with a germinal spot. At a certain 

 stage they escape through the ruptured wall of the ovary 

 into the perigastric cavity. After fertilization they ex- 

 hibit the usual phenomena of segmentation, and are de- 

 veloped into free, ciliated larvae. 



An interesting question arises, as to the mode in which 

 the embryo, imprisoned in the zooecium, makes its escape 

 into the outer world ; and this connects itself with the 

 history of the structure known as the ocecium or ovicell. 

 This is a small, somewhat globose or galeriform receptacle, 

 which in many species of Cheilostomata is developed 

 during the breeding-season, at the upper extremity of the 



