MORPHOLOGY AND SYSTEMATIC POSITION OF SPONGIDA. 107 



Tlie 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 external 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 development is reached we have a fully de- 

 veloped sponge of the type made known by Hseckel 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 phe- 

 nomena observable in Sycandra hold good in all cases. Quite 

 recently the Russian embryologist, M. Ganin,i has given an ac- 

 countj without illustrations, of the development of Spongilla flu- 

 viatilis, which does not appear reconcilable 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 some- 

 M'hat modified condition amongst the Fibrospongise and Myxo- 

 spongiae. 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 ex- 

 plained by regarding it as a transitional form between the Pro- 

 tozoa and Metazoa. According to this view the larva is to be 

 considered as a colony of Protozoa, one half of the individuals 

 of which have become differentiated into nutritive forms, and 

 the other half into locomotor and respiratory forms. The gra- 

 nular amoeboid cells represent the nutritive forms, and the ciliated 

 cells represent the locomotor and respiratory forms. That the 

 passage from the Protozoa to the Metazoa may have been effected 

 by such a differentiation is not improbable on a priori grounds, 

 and fits in very well with the condition of the free swimming 

 larva of Spongida, though another and perha})s equally plausible 

 suggestion as to this passage has been put forward by my friend 

 Professor Lankester.^ 



1 " Zur Entwickelung d. Spongilla fluviatilis," ' Zoologischer Anzeiger,' 

 vol. i, No. 9, 1878. 



2 " Notes on Embryology and Classification." This Journal, Vol. XVII, 



