124 



HARDWICKE'S SCIENCE-GOSSIP. 



least, sexually. Such, then, are the main character- 

 istics of the Infusoria. With such a wide basis as 

 this, the individuals are very varied. Even among 

 unicellular organisms, however, there may be differ- 

 entiation of parts, and we find some with a structure 

 which is complex compared to the lower members of 

 this class. 



Nearly every amateur microscopist is familiar with 

 the beautiful little bell animalcule (Vorticella), 

 common in every brook. This represents one of the 

 higher members of the class. Although it is spoken 

 of as unicellular, yet the various regions of its sub- 

 stance have certain functions. The stalk, which 

 attaches the bell to a weed, consists of a central 

 contractile thread, surrounded by a delicate sheath. 

 Around the summit of the bell a double row of cilia 

 circulate, and these guide food into its mouth. The 

 disc from which the inner row of cilia springs may be 

 withdrawn inside the body, and this is effected by 

 the contraction of a certain part of its substance. In 

 the substance of the bell is a clear space, the walls of 

 which contract and expand, apparently driving fluid 

 contents to different regions of its body, like a small 

 heart. Vorticella is a highly developed Infusorian ; 

 but, on the other hand, it may be mentioned in 

 passing that certain of this class are much more 

 primitive, such as those often called Monads. These 

 are free swimming forms, furnished with one or two 

 long cilia, not furnished with mouth, contractile disc 

 or stalk. 



There is an Infusorian closely related to Vorticella, 

 found attached to aquatic animals, sometimes de- 

 scribed as parasitic. It is called Trichodina, and]one 

 species {T. viitrd) is found on the little black Plana- 

 rian, common in every pond. It may be as well to 

 briefly describe this latter organism, as, though so 

 common, it attracts but little attention to the amateur. 

 It is a little leech-like animal, black in colour, some- 

 what less than a quarter of an inch in length, which 

 glides along the bottom of the pond. Its body is 

 capable of considerable change in shape, now 

 elongating, now shortening itself. Examined under 

 the microscope, it is seen to have no suckers ; cilia 

 on the margin of its body are in constant motion ; 

 the dark alimentary canal may be seen branching 

 through the semi-transparent walls ; along the 

 anterior margin is a row of little black eyes. Planaria 

 sometimes presents a wasted appearance; minute 

 transparent bodies cover its surface. These are not 

 warts, but the little Trichodince, which attach them- 

 selves to its surface. 



Trichodina may best be examined by placing the 

 Planarian to which it is attached under a cover-slip, 

 so as to limit the movements of the latter, which is 

 restless, and will wander about the slide unless means 

 be taken to prevent it. The Infusorian may now be 

 examined with higher power (about a quarter of an 

 inch). Like Vorticella, it possesses a disc which can 

 be inverted, and most are seen in this condition as 



small, transparent, truncated elevations, seen best 

 along the margin of the body of Planaria. At each 

 side of the flattened summit there is a small depres- 

 sion which indicates the position where the disc is 

 turned in, A row of fine long cilia lie closely applied 

 to the body of its host, and every now and then a 

 wave-like movement is seen to traverse through them. 

 These long cilia are the active organs of locomotion, 

 and when these animals swim freely, as sometimes 

 they do, the cilia are seen to be in constant motion. 

 If watched carefully, the creature will be seen to 

 suddenly change its form, and appear somewhat lop- 

 sided ; this is caused by the eversion of the disc. 

 The disc, with a little care, is seen to be fringed with 

 a row of cilia ; it slopes downwards from above the 

 mouth, which is situated on the upper free surface of 

 the bell. The mouth is surrounded by a ciliated 

 peristome or ring, which also surrounds the disc. 

 Thus the arrangement is very similar to what we 

 have in Vorticella. The mouth being situated on the 

 free surface, it seems hardly possible that this creature 

 feeds on the tissues of its host. This, then, is not a 

 case of parasitism, but rather a case similar to that of 

 the pea-crab, which lodges in the shell of the oyster. 

 Trichodina is benefited because of the support, and 

 also perhaps the activity of its "host";* while 

 Planaria is little the worse. 



The stalk on which the bell of Vorticella is borne 

 is contractile, and functions as a muscle. We may 

 compare it to a long muscle in the human body. Its 

 origin or fixed attachment is the plant or weed to 

 which Vorticella is attached ; its insertion, or attach- 

 ment to the movable portion, is into the bell. 

 Contraction draws the bell nearer to the stem to 

 which Vorticella is attached ; or, speaking ana- 

 tomically, the place of insertion nearer the place of 

 origin. To carry out the analogy further, I may say 

 that, just as a muscle may exchange its attachment to 

 a fixed part for its attachment to a movable part, so 

 in Vorticella, when it uses its stalk for a locomotive 

 organ, the attachments of insertion and origin are 

 constantly changing.! But to return to Trichodina : 

 we here find a further differentiation. Some of the 

 little organisms become detached from their host in 

 the course of examination, and can be observed with 

 the under-surface upwards. They then appear 

 circular, fringed v/ith the basal row of cilia which, it 

 is most likely, are actively working, and which have 

 been previously described. In the centre there is a 

 circular hole, and round this a ring of protoplasm. 

 External to this again, short bars of protoplasm 

 radiate, which mark off small areas fading gradually 

 into the general hyaline protoplasm as they pass 



* I believe the terms "guest" and "host" are applied in 

 biology to the parasite and the animal or plant it feeds on. I 

 use the terms here because I can find no others convenient to 

 replace them. 



•f- I have never seen mentioned in any book the fact that 

 Vorticella can walk in this manner ; but that it can and does 

 do so, I can vouch for, having seen one progressing in this 

 singular manner along the cover-glass of a slide. 



