DIVISION OF COLLAR-CELLS OF CLATHEINA CORIACEA. 615 



allied to them in a separate order of Flageilata termed the 

 Binucleata. (The question as to whether or not the Hsemo- 

 sporidia should be inchaded in the Binucleata is one which, in 

 the present memoir, we do not wish to raise or discuss.) A 

 trypanosonie is, in our opinion, a binucleate organism, 

 possessing- a ti'ophic nucleus (N), a kinetic nucleus (?i), and 

 a blepharoplast (basal granule). 



In order to settle these disputed points, more knowledge is 

 required regarding nuclear and other structui-es connected 

 with the locomotor apparatus in other organisins, and in the 

 hope of throwing some light on these questions we have 

 studied the division of the collar-cells of a calcareous sponge, 

 of which preserved material was in the possession of on« of 

 us. A collar-cell, although occurring as tissue-element of a 

 Metazoan organism, is essentially a flagellate organism, com- 

 parable in every way with an individual of the Choano-flagel- 

 lata. It has recently been pointed out by one of us (13) 

 that there are two types of collar-cells in calcareous sponges. 

 In one type, characteristic of the family Clathrinidte, 

 amongst Ascons, the nucleus lies at the base of the cell, far 

 removed from the origin of the flagellum, which arises from a 

 distinct basal granule or blepharoplast situated at the apex of 

 the cell. In the other type, characteristic of the Leucoso- 

 leniidse ainongst Ascons, and of the Heterocoela generally, 

 the flagellum arises directly from the pear-shaped nucleus, 

 which is usually situated in the upper part of the cell, close 

 to the point at which the flagellum emerges from the body of 

 the cell. These two diiferences in arrangement are also 

 paralleled amongst free-living Flagellates, for instance 

 amongst the two genera of Mastigamoebse described by 

 Goldschmidt (4), and there can be no doubt that the con- 

 ditions are perfectly comparable in the two cases- — that is to 

 say, that when the flagellum arises from a basal granule 

 distinct from the nucleus, the basal granules are homologous 

 structures. As the result of our investigations we have ob- 

 tained, as will be apparent in the sequel, evidence of a most 

 convincing kind as to the identical nature of centrosomes and 



