THE WHEEL ANIMALCULES. 



245 



lives in a tube, and has a crown of tentacles on the dorsal surface, the mouth being in the midst of 

 them, and the anus opening far in front on the dorsal aspect. The embryo has a two-ciliated body, 

 and that behind the mouth is produced into several lobes, and fringes the free edge of a broad fold of 

 the back, which arches over the mouth. This young form has been called Actinotrocha. As it grows, 

 a part of the skin of the lower part grows inwards, like a pouch, and becomes connected with the 

 middle of the intestine of the embryo. Then it grows out 

 again and covers the intestine in the form of a loop with it, 

 as a projection. This forms the foundation of the adult 

 form, and the tentacles of the embryo or larvse grow into 

 those of the adult. It is like the larva of an Echinoderm. 



CLASS ROTIFER A. THE WHEEL 

 ANIMALCULES. 



Leeuwenhoek found in the rain water of a leaden gutter . 

 animals which were considered animalcules, about the size 

 of a small grain of sand, and which produced currents in 

 the water by means of slender organs or limbs. These they 

 protruded at pleasure. They had bodies of the shape of a 

 pear, with a short stalk, divided into two tails for fixing 

 them on to objects. The microscope gradually increasing LARVA OF PHASCOLOROMA (A) AXD OF SIPUN- 

 in its powers, observers were able to distinguish vibratile cilia CULUS (") (Afler Ge ^ n ^ r -) 



i T , ., . -, ,i a, copbalic lobe, upper lip: 6, under lip; ', intestinal canal : 



upon a protruded disc-like structure, capping as it were the w , vumufie crown ; m, muscular fibres, 



minute animal, and the optical illusion of a rotatory wheel 



of hairs produced by the uninterrupted succession of the strokes given by the cilia of the disc, caused 

 these beautiful and nearly transparent creatures to be called Wheel Animalcules, or Rotifera. The 

 general surface of the body of the Rotifer is not ciliated throughout, and is made up of a layer of clear 

 transparent chitinous tissue, which even becomes shell-like in some, and is ornamented. It is quite 

 evident, under the microscope, that the body has cross markings and constrictions behind, 

 amounting to imperfect segmentation. In front, or nearer the part out of which pass 

 the discs with cilia the trochal discs the constriction is not usually seen, but transverse 

 markings are often visible, so that the whole may be indefinitely marked with six seg 

 ments. This outside coating may have spines or rigid bristles or hairs on it, and when 

 there is a shell or carapace, this is secreted by the skin or by a special organ. The Rotifera 

 have a digestive apparatus, and the mouth is a funnel-shaped cavity situated in the 

 middle or on one side of the trochal disc ; its walls are ciliated, and at the bottom of 

 it is a muscular pharynx, or mastax provided with a peculiar armature or moving jaws. 

 There are four pieces in the mastax, two side ones, the smaller, and two central, forming 

 the incus. Muscles are attached to the movable mallei, and work them forwards and 

 backwards, so that their ragged free ends work the food on the incus. A short oesophagus, 

 also ciliated, leads to a digestive cavity lined with cells and dilated in front, giving off a 

 large cfecum on either side. Behind, the digestive cavity narrows, becomes intestinal, and 

 may open externally by a passage or vent. In some Rotifers the digestive cavity has no 

 ROTIFER second opening, and is a sac without an intestine, and in the males of some forms there 

 VULGARIS. is no digestive track whatever, a solid cord of tissue existing there. The position of the 

 mouth, close to the trochal disc, enables the cilia of this interesting structure to provide it 

 with food by their lashing and current-making. The cilia of the digestive tract assist, and the 

 morsel is crushed and smashed up, before entering the stomach, by the mastax. 



A spacious cavity exists between the digestive organs and the inside of the skin and sides of the 

 body of the Rotifer, and this is of course a perivisceral cavity. The outer opening or vent (cloaca) of 

 the intestinal canal has a large thin-walled vesicle opening into it, which contracts and dilates 

 regularly. This contractile vesicle has two delicate water-vessels, like narrow convoluted tubes which 

 pass forwards giving off branches, and finally form a maze of tubes in the trochal disc. The branches 

 are open at their ends, and as they are outside the digestive organs and inside the walls of the body 



