Development of the Marine Spoyiges. 331 



There are yet two illustrations of the embryo of Halisarca 

 lobular is to be described ; but as it is our object to proceed 

 direct from the last segmentation of the ovum in this sponge to 

 the formation of the embryo in Halichondria si?nula?is (that 

 is, from the second to the third period of ovular development), 

 we shall have to return to these hereafter. 



Third and Fourth Periods. 



As the third and fourth periods of the development of the 

 sponge-ovum will be followed out in Halichondria simulans, 

 Johnst., it might be as well to briefly premise the following 

 description of this sponge : — 



Halichondria simulans. 



General form digital, cylindrical, solid, branched ; procum- 

 bent, adhering here and there to the rock on which it lies ; .or 

 flat, sessile, and spreading. Colour greyish brown or yel- 

 lowish. Vents sparse, scattered, large. Internal structure 

 composed of sarcode and the usual ampullaceous sacs (PI. XXI. 

 fig. 23), hung upon a reticulated, anastomosing, horny, fibrous 

 skeleton, imbedding one form of spicule only. Spicule acerate, 

 smooth, curved, rather abruptly pointed, often somewhat bent 

 in the middle ; average largest size 8 by 1-1 800th inch in 

 its greatest diameters (fig. 24). Size of entire specimens 

 variable. 



Hah. Marine. 



Loc. Plentifully on the under and overhanging surfaces of 

 rocks. 



This sponge, which is very common on this coast, has been 

 named by Dr. Bowerbank " Isodictya simulans" in one part 

 (vol. ii. p. 308) and " Ghalina simulans 11 in another (vol. i. 

 p. 101), also (ib. p. 277) " Isodictya simulans." At this 

 confusion I do not wonder, since in my proposed division of 

 the sponges generally this sponge would come among the 

 Chalinid^e or third division, and Isodictya in the fifth division 

 or Rayneri^e ; while the distinction between Chalina and 

 Isodictya is only one of degree, viz. the almost total absence 

 of supporting horny fibre in the latter. 



The ova of Halichondria simulans in the second period 

 appear in the form of opaque white grains in the centre or 

 oldest portion of the cylindrical forms, and at the base close to 

 the rock in the spreading forms, just as in Halisarca lobularis. 



By what stages the spheroidal ovum passes into the elon- 

 gated embryonal form, I do not know ; but the ciliated ecto- 

 derm as well as the spicules are already developed in the 



