NATURE AND AFFINITIES OF THE SPONGES. 189 



form illustrated by Fig. I of the plate just quoted. Isolating some half- 

 dozen cell-units from this complete section, as shown at PI. IX. Fig. 2, a 

 monad aggregation is produced that corresponds in a most remarkable 

 manner with the characteristic moniliform colony-stocks of the free-swimming 

 marine collared type Desmarella moniliformis, S. K., represented at PI. II. 

 Fig. 30, a fresh-water variety of which genus has been since figured by 

 Professor Stein under the title of Codonodesmus phalanx. The symmetrical 

 pattern of the ampullaceous sacs just described is, however, by no means 

 persistent. In many sponges a greater or less number of these primitively 

 spheroidal chambers, abutting upon one another, coalesce together, and so 

 form altogether irregularly shaped cavities that may ultimately be of very 

 considerable extent. 



In his ' Spongiologische Studien ' * Metschnikoff has quite recently 

 drawn attention to certain structural elements in Halisarca Dujardinii 

 that have hitherto escaped notice. These structures, upon which he bestows 

 the name of " rosette-cells," consist of small subspheroidal groups of cells, 

 usually eight or sixteen, that are developed independently within the inter- 

 stitial substance of the adult sponge, and also within the central cavity of 

 the free-swimming ciliated gemmules, in this latter case being the product, 

 by segmentation, of the metamorphosed and increeping zooids from the 

 peripheral region. Some of the more characteristic representatives given 

 by Metschnikoff of these rosette-cells are reproduced at PI. IX. Figs. 15-17, 

 the two last of these exhibiting the existence of flagellate appendages. 

 It so happens that these newly-reported rosette-cells supply, most oppor- 

 tunely, an important link in the organization of certain Spongida, recognized 

 by the author some years since, but which, pending the production of 

 corroborative evidence, has not hitherto been brought forward. Closely 

 identical cell-aggregations have been thus observed in a variety of sponge- 

 forms, being found more especially abundant, however, in blood-red examples 

 of the same type, Halisarca Dujardinii, collected on the Jersey coast and 

 examined in the living state in the month of February 1878. At this time 

 of the year none of the characteristic ciliated swarm-gemmules were 

 present ; but in cutting sections, numbers of spheroidal uvella-like groups 

 of typical collared monads were liberated, and were likewise observed 

 united to the cytoblastema, entering largely, in combination with the ampul- 

 laceous sacs, under such conditions into the formation of the general sub- 

 stance of the sponge-stock. While in their- most typical and matured con- 

 dition the constituent zooids of these independent rosette-shaped groups were 

 provided with collars and flagella, and in all respects resembled those 

 lining the ampullaceous sacs, in less matured examples they were simply 

 flagellate, and in still earlier conditions possessed no appendages whatever, 

 the larger examples being then indistinguishable from the morula-like cell- 

 aggregates out of which the ampullaceous sacs are themselves developed. 

 There can be but little doubt that these structures observed in the Channel 



* 'Zeitschrift flir Wissenscha'tliche Zoologie,' Bd. xxxii., 1879. 



