PKOPAGATION OF SEKTTJLABIAN ZOOPHYTES. 97 



(230.) The third mode of multiplication, or that by Planulce, seems to 

 be specially adapted to the diffusion of the species ; and as it presents 

 many points of peculiar interest, we shall dwell upon it at some length. 

 At certain periods of the year, besides the ordinary cells adapted to con- 

 tain nutritive polyps, others are developed from different parts of the 

 stem, which may be called female or fertile polyps, although usually 

 simply termed the vesicles. The cells of this kind are much larger than 

 the nutritive cells, and of very different form (fig. 48, A) ; they are 

 moreover deciduous, falling off after the fulfilment of the office for which 

 they are provided. They are produced in the same manner as the rest 

 of the stem, by an extension of the tegumentary membrane (fig. 45, 5), 

 which, as it expands into the form of the cell, becomes of a horny 

 texture ; it may be traced, however, over the opening of the cavity, 

 where it sometimes forms a moveable operculum. The cell being thus 

 constructed by the expansion and subsequent hardening of the tegu- 

 mentary membrane, it remains to explain the origin of the reproductive 

 germs which soon become developed in its interior. 



(231.) According to the observations of Loven on the reproductive 

 process in Sertularia, the first appearance of the reproductive germ is a 

 slight elevation (derived from the central mass contained in the ovarian 

 vesicle), in the centre of which an active circulation of nutritious 

 globules seems to be concentrated. This protuberance gradually enlarges 

 and assumes a spherical form, the part whereby it is attached to the 

 central mass becomes constricted, and at the same time its cavity becomes 

 enlarged, and divided into several compartments. 



(232.) Upon the outer aspect of the newly-formed germ a little 

 spherical body may be detected, composed of coloured granular sub- 

 stance, in which a circular transparent spot speedily becomes per- 

 ceptible. 



(233.) A delicate translucent capsule envelopes the parts described 

 above, which, after a time, exhibits at its upper and outer surface a 

 circle of minute elevations. This capsule Loven regards as the body 

 of a female polyp, of which the little elevations are the rudimentary 

 tentacula ; and its contents manifestly constitute an ovum, enclosing a 

 Purkinjean vesicle. Several of these ova are formed in the ovarian 

 vesicle, presenting different degrees of development, the upper ones 

 being the most advanced in growth. In proportion as each ovum in- 

 creases in size, the original sacculus, which is merely a prolongation of 

 the central living substance of the polypary, and which at first formed 

 the larger part of the germ, becomes proportionally smaller, owing 

 to the rapidly-increasing dimensions of the ovum, and soon the vesicle 

 of Purkinje is no longer discoverable. Meanwhile the canal whereby 

 its cavity communicates with the central mass becomes elongated; so 

 that its union with the common substance of the polypary is not de- 

 stroyed even when the "female polyp " has burst through the external 



