152 



LECTURE VIII. 



grapes in clusters by tlie pellucid (chorionic ?) membrane in which 

 they are enclosed {Ji(/. 73., 1.) ; tlie yolk is a "germ-yolk," and has a 

 ;o vitelline membrane separated 



by a whitish fluid from the 

 chorion. The fission of the 

 germ-cell is followed by 

 total cleavage of the yolk, as 

 in 2 ; it next subdivides into 

 four, as in 3 ; after which the 

 formation of the germ-mass, 

 4, proceeds rapidly. Its sur- 

 face becomes smooth, large 

 cilia are developed from one 

 end, which becomes marked 

 oif by a constriction from the 

 Development of Pediceiiina. other, as at 5. The larva 



escapes from the chorion under the form 6, and swims freely abroad. 

 The ciliated margin expands, and renders that end of the larva 

 funnel-shaped. Tubercles bud forth from the funnel, and a pedicle 

 is developed from the opposite end, as at 7 ; by this the larva attaches 

 itself, and in the course of the subsequent metamorphosis the linea- 

 ments of the parent Bryozoon soon begins to be traceable, as at 8. 



The development and vital phenomena of the reproductive gem- 

 mules have been studied by Dr. Farre with much care and success in 

 the sponge-like Halodactylus (^Alcyo7iium gelatinosunij of Pallas). 

 They appear in spring as minute whitish points just below the surface. 

 If one of these points be carefully turned out with a needle, it is 

 74 found to consist of a transparent sac, containing 



generally from four to six of the larvas. These 

 are of a semi-oval form, with the margin of their 

 plain surface developed into tubercles support- 

 ing groups of vibratile cilia {fig. 74.).* The 

 body presents a simple granular structure ; the 

 gemmule swims about actively by the vibration 

 of its cilia, the motion of which seems to be 

 under its control. They generally swim with 



Larva of Halodactylus, from ^, n t . i 



above; the cilia as when slowly the couvcx part lorwards ; somctimcs tliev 



acting round the margin in • , , . . 



waves. Simply rotate upon their axis, or execute a 



series of summersets ; or, selecting a fixed point, they whirl round 



* The movements of the cilia, which give the appearance of a succession of 

 Avaves, have been closely analysed in XXXV. pp. 410, 41 1. 



