216 COMPARATIVE ANATOMY 



Other individuals of much larger size, measuring as much as 

 o'i2 mm. in diameter, are situated in the axils of the branches 

 and are distinguished as the macrogonidia. They are globular 

 in shape, provided with a nucleus, contractile vacuole, and a 

 posterior circlet of cilia, but their peristomial field is permanently 

 closed. 



The arborescent form of a colony of Zoothamnium is due to 

 the fact that longitudinal binary division of the ordinary in- 

 dividuals involves not only the body but also the distal half 

 of the contractile filament, and consequently repeated binary 

 divisions give rise to a dichotomously-branched colony. But, 

 since both products of a division remain fixed on the branched 

 stalk instead of one of them swimming away, there would be 

 no provision for the foundation of new colonies unless some 

 means were especially adapted to that end. These means are 

 supplied by the macrogonidia, whose sole function it is to 

 reproduce new colonies. It is not quite certain whether a 

 macrogonidium must undergo fertilisation before it reproduce 

 a new colony or not. At any rate it may undergo fertilisation, 

 that is to say, it may conjugate with a microgamete. The 

 microgametes of Zoothamnium, so far as is known, are produced 

 by multiple division of an ordinary individual, and they have 

 much the same appearance as the microgametes of a Vorticella. 

 They swim away and conjugate with a macrogonidium, never 

 with an ordinary individual. When conjugation is effected 

 the fertilised macrogonidium is liberated from the colony, 

 swims about for a while, and then attaches itself and divides 

 rapidly and repeatedly, giving rise to a new colony. It also 

 seems to be possible that a macrogonidium may be able 

 to detach itself and give rise to a new colony without 

 fertilisation. 



An interesting comparison may be drawn between Zootham- 

 nium and Volvox. In both these forms, though they belong 

 to different classes of the Protozoa, the cell individuals formed 

 by successive binary divisions remain in organic connection 

 with one another and give rise to a compound organism, a 

 stock or colony. In Volvox the cell units are so intimately 

 connected that their individualities are to a great extent merged 

 into the higher individuality of the stock of which they form a 

 part, whilst in Zoothamnium the union is of a looser kind, and 

 the individualities of the cells seem to us to preponderate over 



