ESCAPE OF A PLANULE. 815 



dently perforate, through which the commou core 

 communicates with the yelk until its develoj^ment into 

 a planule. None that I saw contained more than a 

 single ovum, which is large, regularly oval, and of a 

 rich huff-yellow hue, conspicuous to the naked eye, 

 especially when there are many crowded on the stalk. 

 This ovum gradually developes itself, within the vesicle, 

 into a pear-shaped planule, covered with minute cilia 

 in every part, the vihratile action of which can be de- 

 tected by the microscope before its extrusion. As its 

 escape draws near, it slowly moves about in its prison, 

 lengthening and shortening its body, and slightly 

 altering its outline in different parts. 



I observed a planule (and afterwards another) in 

 the act of escaping from the vesicle ; the animal was 

 sluggish and the process tedious, as compared with 

 the emergence of the planule of Plum, setacea. This 

 may perhaps be accounted for by the extreme minute- 

 ness of the cilia in the Antennularia, they being very 

 much smaller than in Pliimiilaria, though the pla- 

 nule itself is three or four times as large. The body 

 was constricted by the margin of the vesicle as it 

 slowly emerged ; but it was not until the posterior ex- 

 tremity was quite freed that I perceived the presence 

 of an unsuspected impediment. The vesicle is fur- 

 nished with a lid, fastened like the cover of a box, at 

 that side which looks towards the stalk of the poly- 

 pidom, by a sort of hinge ; it was forced open by the 

 emerging planule, and fell down nearly close (see 

 Tig. 6), as it finally quitted, by the elasticity of the 

 hinge. 



The freed planule is about ^r-th inch in length, but 



