H. JAMES-CLARK ON THE AFFINITIES OF SPONGES. 325 



The oslioles (fig. 64, o), or incurrent channels, are very numerous, there being at least 

 two, and often three, opposite every interstice of the spicula. They are very small, 

 but quite conspicuous, especially at their inner ends, where they plunge through the 

 monadigerous layer (md). They afford great assistance whilst studying the contractile vesi- 

 cles, and the action of the flagella, since they enable one to get a freer view of the monads, in 

 an undisturbed state, than where they are observed through all the tissues. It should be men- 

 tioned, however, in this connection, that the profile view (fig. 41) of the monads was obtained 

 by making an actual section of one of the younger branches and allowing it to revive and 

 expand in a fresh supply of sea-water. 



The monadigerous layer (figs. 41, 64, md) lines the cavity of the body; and it is by the 

 combined action of the vibrating flagella (fl) of the monads that currents of water and 

 floating particles are kept up. This layer is composed of monadiform animalcules (md), 

 packed closely side by side in a vast colony, which extends over the whole length and 

 breadth of the general mass. In this respect we are reminded of the similar arrangement 

 of the individuals of that floating ascidian Pyrosoma. These monads crowd so closely 

 upon each other that their sides are mutually compressed, and they thus form a sort of 

 irregular, polygonated pavement (fig. 64, md). They all lie with the anterior end (fr) 

 turned inwardly, and projecting into the general cavity, and the posterior extremity 

 imbedded in the cytoblastematous external, general envelope. 



The body of a monad is yellow when seen by transmitted light, and in general terms may 

 be designated as broadly oval, with the longer axis extending antero-posteriorly. Behind 

 it is broadly rounded ; at the sides lightly indented and irregularly polygonal by mutual 

 contact with others; and extended in front into a delicate, membranous, circular collar (b, b l ), 

 which might be compared to a transverse section of a tube which is about as long as it is 

 broad. This collar is capable of variations in form, like that of Codosiga (§ 6) and Salpmgwca 

 (§§ 7, 8, 9), at one moment assuming a truncate conical shape (figs. 43, 44, b), and in the 

 next' instant expanding its distal margin into a distinct flare (fig. 42, b) which is at least two 

 thirds as wide as the body ; or, finally, it retracts altogether and disappears for a while, but 

 eventually reappears, and expands to its fullest dimensions. 1 



The flagellum (fl) is the only prehensile organ which the monad possesses. It arises from 

 the middle of the frontal area and extends to a great length, at least five or six times as long 

 as the body, with scarcely any diminution in thickness. It is a comparatively thick fila- 

 ment, and quite conspicuous ; on which account it is so easily seen through the whole mass 

 of the colony. It usually vibrates with considerable vigor from base to tip, but occasionally 

 assumes the quiescent state, and arcuate form so eminently characteristic of that of Codo- 

 siga (§ 6), Bicosceca (§ 3), and others. 



l In this connection it may be well to mention the latest side of the root of the cilium." If now we suppose these " two 



decision of Carter in regard to the structure of the monocil- spines," to be the right and left profiles of a membranous, cylin- 



iated sponge-cell. In the Annals and Magazine of Natural drical collar, such as I have described in Leucosolenia, then it 



History, Vol. xx. 1857, PI. 1, figs, to, 11, this cell is repre- follows that the monociliated sponge-cell of Sj,<mr;il!a is like 



sented 'as an oval body, with a single ciliary appendage ; but that of the former. That Carter did not always find these 



in a subsequent communication to the same periodical (Vol. iii. "two spines," may be explained by the fact that the mem- 



1859, p. 14, PI- 1. figs- 12, 13, 14,) a partial recantation seems branous collar, as 1 am inclined to believe the " spines" to be, 



to be made, and the cell in question is figured with » two was retracted ; since I have frequently observed this to happen 



spines or ear-like points projecting backwards, one on each in the case of Leucosolenia, when it was disturbed. 



MEMOIRS HOST. SOC. NAT. HIST. Vol. I. Pt. 3. 6 



