Characteristics of Gregarinae, clxvii 



which however do not lose the power of reappearing as separate 

 organisms. The ciliated cells line the interior of certain spheroidal 

 chambers in the substance of the Sponge ; and by their action water 

 is drawn in through the smaller ' pores/ and with it the alimentary 

 particles which the amoebiform cells appropriate. The larger ori- 

 fices or 'oscula^ are exhalant in function. Both sets of orifices are 

 opened or closed at the will of the animal ; but the larger are 

 permanent, whilst the smaller are intermittently formed and in- 

 termittently obliterated. A wide interspace intersected by irre- 

 gular trabeculae may be interposed between the exterior layer of 

 the Sponge in which these orifices are situated, and the deeper 

 layers which make up the great mass of the organism. The spi- 

 cula have a cavity in their interior, which is occupied by sarcode, 

 and are thus to be considered as produced like ^ external skeletons^ 

 or ' tests' by an outward excretion-process. 



Reproduction may be sexual, ova and spermatozoa being produced 

 by the specialization of cells in the general parenchyma ; or asexual, 

 in the way of fission, or in that of the production of gemmules. 

 The gemmules of the fresh-water Sponge are formed towards the 

 close of the warmer period of the year by means of the 'encys- 

 tation"* of portions of the parenchyma, whilst the sexual process 

 takes place during the summer months ; so that the history of the 

 ' winter'' and 'summer' ova of the Daphnidae and Rotifera is here 

 exactly reversed. 



Class, Gregarinae. 



Protozoa, with an external envelope only obscurely limited off 

 from the contained parenchyma, without either mouth or anus, of 

 parasitic habits, moving in some cases with considerable energy, not, 

 however, by the protrusion of pseudopodia, but by the contraction of 

 their ordinarily vermiform bodies in a direction from behind for- 

 wards. They are very ordinarily visible to the unassisted eye, 

 presenting, when circular in shape, the appearance of small white 

 sj)ecks, which may be as much as a millimetre in diameter, but 

 which attain sometimes a length of as much as half an inch, when, 

 as more usually, they are elongated and vermiform. 



They may have the appearance of being divided {Dicystidea) into 

 two unicellular organisms : the septum, however, is stated by Kolli- 

 ker to be produced, not by an involution of their external cell-wall. 



