WORMS. 



205 



queer Asplanchnid^, remarkable for having one opening of the digestive canal ; but 

 the majority of the wheel animalcules, which the collector obtains, belong among the 

 relatives of the Rotifer vulgaris, yet present to our inquisition a great variety of 

 shapes and diversity of habits. They have been divided into three families : First, 

 the Philodinidse, of which the genus Rotifer is the type; second, Brachionidae 

 (Rrachionis, JVbteus, Mdstigocerca, Stephanops, etc.) ; third, Hydatinidse {Hydatina, 

 Monocerca, Wotommata, Diglena, etc.). We shall give an account of a typical species, 

 of the second and third family each, 

 having already described the type of 

 the first. 



In the BEACHioNiDiB the body is 

 broad, almost shield-like sometimes, 

 the foot is short and jointed, and the 

 integument is, in part, so hardened as 

 to create a so-called carapax. JVbteits 

 quadricornis is perhaps the best ex- 

 ample of the family. The broad rump 

 is covered by the shell, which has 

 many nodules upon its surface, and is 

 prolonged into four horns, — two by 

 the head and two near the tail. The 

 wheel forms a notched cup, and serves 

 to sweep food down to the mouth at 

 the bottom of the cup, as well as to 

 act as an organ of expeditious loco- 

 motion. 



Hydatina senta is a classical ani- 

 mal, because it was principally on this 

 species that the illustrious? Ehrenberg 

 studied the anatomy of this group of 

 animalcules. The broad body has only 

 a very short foot-stalk, which is forked 

 behind. The mouth is armed with 

 two jaws and many teeth. There are 

 no eye-specks whatsoever. The cuticle 

 is delicate and soft. Hydatina illus- 

 trates the extreme rapidity with which 

 the development of the young is ac- 

 complished. The egg is extruded 

 within a few hours after the rudiment 

 of it becomes distinctly visible. Within 

 young animal comes forth 



Fig. 190. — Noteus quadricornis. 



twelve hours more the shell bursts and the 

 In Rotifer and several other genera the young are extruded 

 alive, incubation being completed within the parental body. In Rrachionus still a third 

 method prevails, the eggs remaining after their extrusion attached to the posterior end 

 of the body, until the young are set free. Ehrenberg has made the astounding estimate 

 that Hydatina may multiply so rapidly that one hundred millions may be produced 

 within ten days from a single individual ! I confess that this appears to me a totally 

 erroneous calculation, resting upon an utter misapplication of the multiplication table. 



