as Infusoria flagellata. 207 



stratum, the faint yellow spicula (fig. 64, sp,sj>^) are imbedded 

 in systematic order, and overlap each other irregularly in two 

 or three layers. They present two diverse forms — namely, a 

 simple aciculate shape (sj)^) and a stellato-triradiate (sp) one. 

 The rays of the latter are slender, tapering frequently to a 

 bifid termination, divergent at equal angles from each other, 

 and lie in the same plane. Witliout exception they are all 

 arranged with one ray (often longer than the others) projecting 

 backwards, t. e. away from the excurrent orifice, and the other 

 two extending symmetrically right and left, and obliquely 

 transverse to the longer axis of the branch. In this manner 

 they are disposed in a sort of network over the whole colony, 

 even close up to the excurrent orifices ; and as the aciculate 

 spicula lie parallel with the rays of the other kind, there are 

 consequently no projecting spines specially devoted to guard- 

 ing the entrance to these apertures. 



The ostioles (fig. 64, o), or incurrent channels, are very nu- 

 merous, there being at least two, and often three, opposite to 

 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 afibrd great as- 

 sistance whilst studying the contractile vesicles 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 mentioned, however, in 

 this connexion, 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 sup- 

 ply 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 {fi) 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 ge- 

 neral mass. In this respect we are reminded of the similar 

 arrangement of the individuals of that floating Ascidian, Pyro- 

 soma. 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 



15* 



