7 6 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



of the parental body which projects either outward from the free surface 

 or inwards into a special brood-cavity in which it remains for some time 

 before being set free (Fig. 30, B). 



Again the process of syngamy in the life-history of the Acinetaria 

 agrees closely with that of the typical Ciliata. 



It is interesting to note that some of the Acinetarians have taken 

 on a parasitic mode of life : sometimes they are parasitic in the adult 

 condition on Hydroids (Ophryodendron on Marine Hydroids) ; more 

 commonly the young stage alone is parasitic, burrowing in some cases 

 into the body of other Acinetarians (Tachyblaston in Ephelota). An 

 intermediate step towards the parasitic condition is shown by numerous 

 genera whose habit is epizoic, i.e. which live attached simply to the 

 surface of other aquatic animals such as Hydroids, Crustacea, etc., 

 without actually absorbing nourishment from them. 



Before leaving the subdivisions of Protozoa it is necessary to glance 

 at two groups of organisms the relationships of which are rather uncertain 

 but which have come into prominence of recent years in connexion with 

 the causation of disease. 



THE SPIROCHAETES 



The first of these includes a variety of creatures which may be called 

 by the general name Spirochaetes. These resemble somewhat in general 

 appearance very slender Trypanosomes and although formerly grouped 

 together as one genus Spirochaeta are now usually subdivided among a 

 number of separate genera. For the sake of simplicity we shall here 

 adhere to the older practice mentioning parenthetically the newer 

 generic names. 



A spirochaete is as already indicated somewhat like a very slender 

 and usually very minute trypanosome. The thread-like body shows in 

 the dead specimen a number of undulations from side to side but owing 

 to the minute size it is difficult to be certain as to whether these are 

 merely bends from side to side as in a trypanosome or, as is more probable, 

 the turns of a cork-screw spiral. When alive the movements are very 

 characteristic, the spirochaete swimming rapidly in the direction of its 

 length for a short distance and then reversing its movement. 



It is again difficult to make sure whether this movement consists, 

 like that of a trypanosome or an eel, of movements of flexure from side to 

 side, or merely of a rotatory movement of the spiral about its long 

 axis. 



