NATURE AND AFFINITIES OF THE SPONGES. 1 85 



exhibits various phases of unevenness which may be thus explained. In all 

 three, the posterior half has developed considerably in advance of the 

 anterior one, but exhibits in the first figure its typical composition of collar- 

 bearing monads ; in the other two, on the other hand, the posterior basal 

 elements have passed from the collar-bearing to the amceboid state, and 

 coalesced more or less extensively wdth each other. In a like manner, the 

 elements of the anterior half in both of the Figs. 26 and 27 represent the 

 immature and simply flagellate phases of the collared zooids, but which in 

 Fig. 28 have acquired their characteristic adult form. The life-history and 

 various developmental phases of the free-swimming ciliated sponge-gem- 

 mules are, in fact, epitomizations only of the phenomena already described 

 of the collared zooids of the adult sponge-stock. Here, as there, the 

 essential constituents, or collared zooids, of the colonial aggregation, com- 

 mence life as simple flagellate units, which, after attaining their typical 

 adult form, assume successively an amoeboid and quiescent state, and give 

 rise by minute sporular subdivision to a further progeny of collared zooids. 

 As a clear indication of the common origin and significance of both the 

 moruloid and amphiblastuloid sponge-gemmules, with their various modifi- 

 cations, it needs only to be recorded that every one of these seemingly 

 distinct structures has been met with by the author in a single sponge- 

 stock of Grantia compressa, and also in Leucosolenia botryoides and Grantia 

 {Sycon) ciliatum, as obtained both in the Channel Islands in the years 

 Y^yj-^, and on the South Devon and Cornish coast in 1879. From 

 the last examined examples, furthermore, sections containing these 

 gemmules in abundance, treated with osmic acid, have been successfully 

 preserved for permanent reference and comparison. It is, moreover, 

 in connection with the first-named sponge-form {Grantia compressa) 

 that Barrois has reported the occurrence of that variety of the amphi- 

 blastuloid gemmule, in which a ring of intermediate non-flagellate cells 

 are developed equatorially between the typical flagellate and amoeboid 

 series. The latter elements, as shown at PI. IX. Fig. 30, are of somewhat 

 abnormal size, and, as in many other of the examples figured, are far too 

 large to have represented individually a primarily single collared zooid. 

 This phenomenon, as also that of the presence of the equatorial girdle of 

 smaller cells in the example cited^ admits of simultaneous explanation. 

 The equatorial girdle, in fact, may be interpreted as representing an anterior 

 ring of amoeboid cells metamorphosed from the typical collared units at a 

 slightly later date than those of the posterior area, and which latter have 

 become still more obscurely transformed and increased in size, though 

 lessened in number, by coalescence. It is by a similar process of retro- 

 gression to an amoeboid type, and the more or less extensive union or 

 coalescence of amoeboid zooids that the ciliated gemmules themselves 

 originate. 



In addition to the various symmetrically constructed moruloid and 

 amphiblastuloid modifications of the ciliated sponge-gemmule already 



