of Mollusks in Holothurise. 27 



more richly developed, and here form complete seiies in the 

 cells. Similar cells, however, everywhere form the outer layer. 

 Below these lies the muscular membrane, consisting of transverse 

 and longitudinal fibres, on which the slow worm-like movements 

 of the sac, which may commonly be observed, depend. Internal 

 to this muscular coat perfectly transparent cells are scattered. 

 The innermost layer is formed by a membrane which presents a 

 lively ciliary motion everywhere — save in the green part. It is 

 in this ciliated part of the sac that the ovarium and the sperm- 

 sacs lie perfectly free, like the charge in a gun. 



The ovary again has its peculiar investment. This is a long 

 tubular capsule, everywhere closed and ciliated upon its whole 

 surface. The ciliated inner surface of the sac therefore is turned 

 towards the ciliated outer surface of this capsule of the ovary. 



The ovarian capsule is for the most part, but not wholly, filled 

 by the ovary. The end which is turned towards the intus-sus- 

 ception of the sac is always empty and much thinner than the 

 other part of the capsule, and this thinner end is always bent 

 back upon itself, so that not its end, but its knee-shaped bend, is 

 in contact with the extremity of the green intus-susception. 



It is here that the structure of the capsule may best be made 

 out. Beside the ciliated membrane there is an internal layer of 

 small elongated cells with scattered clear globules about y^o*^ 

 of a line in length. In all the rest of the capsule, so far as it is 

 filled by the ovary, scattered agglomerations of yellow fat gra- 

 nules may be perceived upon its inner surface between it and the 

 ovary. 



The orange-coloured ovary lies in the capsule, but does not fol- 

 low its shape. It is dendritic, so that the capsule passes over the 

 branched mass ; and in an ovary which is not quite fully-formed, 

 little clear gaps may be observed here and there between the 

 branches of the ovary. The dendritic figure is that of a central 

 stem, from which branches are given off on each side and divide 

 and subdivide. 



The central stem lies against the inner surface of the ovarian 

 capsule, so that in the imperfectly developed ovary, a hollow 

 space exists within the capsule which disappears in the course of 

 further development. If the capsule be carefully opened, the ova- 

 rium may be extracted and its structure further examined. It 

 consists, in all its parts, of egg-like masses -Jyth of a line in dia- 

 meter, inclosed in membranes ; the contents are coarse stearine- 

 like yelk- granules (of 2^^^ to ^^^j of a line in diameter), and a 

 germinal vesicle of jV^h of a line in diameter without a germinal 

 spot. The germinal vesicle is viscid throughout, and is more similar 

 to what Von Baer calls the " nucleus of the ovum ^' in the perfect 

 ova of Echinus. The yelk-granules are frequently aggregated 



