DEVELOPMENT OF HALISAEOA LOBULARIS. 611 



stage Schulze's observations, like those of Barrois, were made 

 on free- swimming larvse; indeed, in the case of Schulze's 

 specimens development does not proceed within the parent 

 Sponge beyond the formation of a blastula, or, at all events, 

 of an incipient gastrula. Save for the exceptional mode of 

 formation of the blastula our discordant results might then be 

 explained as due to the fact that mine are obtained from intra- 

 uterine embryos, as one might say, while Schulze^s were made 

 on free-swimming embryos. But this leads to a further 

 inquiry, that is, for what reason should the embryos of the 

 Roscoff Sponges remain up to later stages within the maternal 

 tissues than those of the Sponges which Schulze obtained at 

 Trieste ? In reply to this, I would inquire whether it is not 

 possible, indeed highly probable, that an explanation is to be 

 found in the wide differences of condition which characterise 

 the two localities whence the Sponges were obtained. In the 

 Mediterranean there are no heavy tides and powerful currents, 

 so that the Sponge larvae can safely issue iuto the warm sur- 

 rounding water at an early stage without fear of being washed 

 away to sea out of reach of a holdiug-place, and in this early 

 birth there is a distinct advantage to the race, for the larvee 

 can obtain as much oxygen and nourishment as they require 

 from the surrounding water without putting their parent to 

 the needless expense of maintaining them. In the English 

 Channel, particularly about RoscofF, where the tides run high 

 and the tidal currents are exceedingly rapid and dangerous, 

 the case is very different ; young larvae set free too soon would 

 be swept away from home and drifted up and down by the 

 currents with the merest chance of finding a safe settling-place, 

 and hence it becomes necessary that they should be prepared 

 to attach themselves to a foreign object immediately after 

 hatching as possible. Consequently in Sponges obtained from 

 this locality we can trace their development up to a stage 

 which is evidently not far remote from that of a young Sponge, 

 and in the superfoetation thus induced by external conditions 

 we find the key to the anomalous development we have de- 

 scribed. 



