I 88 NATURE AND AFFINITIES OF THE SPONGES. 



produced first in the deeper and consequently older portion of the sponge- 

 stock, and that where abundantly developed they monopolize its interstitial 

 substance to the exclusion of the ampullaceous sacs or other combinations 

 under which the collared zooids may be characteristically distributed. 

 These latter have, in fact, after attaining maturity, assumed the amoeboid 

 state, and, abandoning their normal position, coalesced extensively with 

 one another after the manner of various ordinary Flagellata, the outcome 

 of this process being the more or less regular segmentation of the united 

 mass and production of the characteristic ciliated gemmule. Regarded 

 from such a point of view, this ciliated reproductive structure is in no sense 

 an egg, or its derivative, but represents a coherent aggregate of monadiform 

 swarm-spores, or, as it may be most appropriately denominated, a " swarm- 

 gemmule." 



In addition to the characteristic ciliated reproductive bodies or swarm- 

 gemmules just described, there remain to be discussed certain other complex 

 bodies, generated within the interstitial sponge-substance, that, as first 

 suggested by Mr. Carter, take their origin by an essentially identical 

 developmental process. The structures here referred to are the so-called 

 " ampullaceous sacs " or spheroidal ciliated chambers characteristic of Hali- 

 sarca, the greater portion of the keratose and siliceous sponge-forms, and 

 some few calcareous species. It has been ascertained by the author in the 

 case of Halisarca Dujardinii, a species of Isodictya, and various other types, 

 that these structures are also, as shown at PI. IX. Figs. 4-9, originally 

 developed from the segmentation of a primitive amoeboid body produced, 

 as in the former instance, by the coalescence of more or fewer metamor- 

 phosed collared zooids. Such segmentation is in this instance, however, 

 entirely even, and results in the production of a perfectly spheroidal moruloid 

 body having a somewhat extensive central segmentation cavity. The only 

 difference that characterizes the more advanced condition of the ampul- 

 laceous sacs is manifested by the fact, that while in the case of the free- 

 swimming ciliated gemmules the constituent collared zooids are so de- 

 veloped that their collars and flagella are directed peripherally or away 

 from the central segmentation cavity, in the case of the ampullaceous sacs 

 they take an opposite direction, their flagella and collars being projected 

 into this cavity. As with the ciliated gemmules, the component zooids, 

 before acquiring their characteristic collars, present a simply flagellate con- 

 dition, their aspect at such stage being represented at PI. IX. Fig. 11. 

 Immediately prior to this simply ciliate condition the individual units are 

 amcebiform and non-flagelliferous, and held together circumferentially by 

 a thin film of structureless cytoblastema ; often, as shown at PI. IX. Fig. 10, 

 they are considerably isolated. In their more matured state the collared 

 zooids cohere laterally to one another, and, excepting at the afferent and 

 efferent apertures, present no break or interruption. Taken in optical 

 section, so as to avoid these openings, a matured and typical ampullaceous 

 sac presents, in fact, the exceedingly elegant and symmetrical structural 



