Development of the Marine Sponges. 325 



In studying the soft parts of a sponge, including the ovule, 

 it is also essential to remember that they are all polymorphic, 

 and, like Amoeba, may at one moment appear in one form and 

 at another in a different one. Thus the active spongozoon, 

 which in situ may present a defined body, neck, head, and 

 cilium, may, shortly after having been torn out from its 

 natural position, and thus rendered passive, be changed into a 

 simple, globular form. This form, in returning to partial 

 activity, may again throw out pseudopodia from its sides 

 (Amoeba-like) and become reptant. The cilium, when the 

 spongozoon is in situ, is used for bringing objects to the body ; 

 when the spongozoon is isolated it becomes an organ of loco- 

 motion, by which the body is propelled in front of it ; from 

 this state it may pass into a pseudopodial, prehensile form, 

 and finally be retracted altogether ; while the ovum, from its 

 first appearance until after it has attained a considerable size, 

 is always provided with a sarcodal envelope which, Amoeba- 

 like, gives it (up to a certain stage of development) a loco- 

 motive power. The student, therefore, must not be surprised, 

 by-and-by, to find the embryo, after this manner, losing at 

 once the cilia of its surface-layer of cells (the ectoderm), and 

 the latter becoming a homogeneous-looking polymorphic or 

 Amoeba-like lamina of sarcode. 



When the ovule for the first time becomes recognizable in 

 Halisarca lobularis it does not exceed the 1 -3000th inch in 

 diameter, and then appears to be confined to the tissue of the 

 sponge among the ampullaceous sacs (PL XX. fig. 3, a). Its 

 envelope, however, already possesses the power of polymorphism 

 and locomotion, as may be seen when it is scratched out from 

 the parent upon a slide. 



Subsequently, in Grantia compressa, it may be seen to be 

 hanging, pear-shaped, upon the surface of the excretory canals, 

 where it remains for a certain time locomotive, until, after 

 further development, it becomes permanently fixed and the 

 locomotive envelope seems to pass into a capsule. In this 

 condition, too, I have described and figured it in Tethya zet- 

 landica ( l Annals,' 1872, vol. ix. p. 426, pi. xxii. fig. 14). 



Although by necessity spread throughout the walls of the 

 purse-like forms of calcareous sponges (ex. gr. Grantia com- 

 pressa), it is nevertheless developed in Halisarca lobularis and 

 Halichondria simulans (with which again in form the sessile 

 spreading species of calcareous sponges, ex. gr. Leuconia 

 nivea &c, are identical) close to the rock on which the sponge 

 may be growing, where it becomes heaped up into masses, 

 which present the ova in all stages of colour and develop- 

 ment, from the first degree of duplicative division of the yelk 



