POSITION OF THE SPONGIDA. 665 



though surrounded by a ring of four-rayed spicules is itself free 

 from them. At this extremity a small perforation is formed leading 

 into the gastric cavity which rapidly increases in size and forms 

 an exhalent osculum (os). A series of inhalent apertures are 

 also formed at the sides of the cylinder. The relative times of 

 appearance of the single osculum and smaller apertures is not 

 constant for the different larvae. On the central gastrula cavity 

 of the sponge becoming placed in communication with the ex- 

 ternal water, the entoderm cells lining it become ciliated afresh 

 (fig. 3, B, en] and develop the peculiar collar characteristic of the 

 entoderm cells of the Spongida. When this stage of develop- 

 ment is reached we have a fully developed sponge of the type 

 made known by Haeckel as Olynthus. 



Till the complete development of other forms of Spongida 

 has been worked out it is not possible to feel sure how far the 

 phenomena observable in Sycandra hold good in all cases. 

 Quite recently the Russian embryologist, M. Ganin 1 , has given 

 an account, without illustrations, of the development of Spongilla 

 fluviatilis, which does not appear reconcileable with that of 

 Sycandra. Considering the difficulties of observation it appears 

 better to assume for this and some other descriptions that the 

 observations are in error rather than that there is a fundamental 

 want of uniformity in development amongst the Spongida. 



The first point in the development of Sycandra which deserves 

 notice is the character of the free swimming larva. The peculiar 

 larval form, with one half of the body composed of amoeboid 

 granular cells and the other of clear ciliated cells is nearly con- 

 stant amongst the Calcispongise, and widely distributed in a 

 somewhat modified condition amongst the Fibrospongiae and 

 Myxospongiae. Does this larva retain the characters of an 

 ancestral type of the Spongida, and if so what does its form 

 mean ? It is, of course, possible that it has no ancestral meaning 

 but has been secondarily acquired ; I prefer myself to think 

 that this is not the case, more especially as it appears to me that 

 the characters of the larva may be plausibly explained by 

 regarding it as a transitional form between the Protozoa and 

 Metazoa. According to this view the larva is to be considered 



1 " Zur Entwickclung d. Spongilla fluviatilis," Zoologischer Anzeiger, Vol. 'l. 

 No. 9, 1878. 



B. 43 



