WORMS. 



205 



qut'cr AsPLANCiiNiDvE, 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 

 ;<hapes and diversity of habits. They have been divided into three families : First, 

 the Philodinida;, of which the genus Rotifer is the type; second, Brachionidaj 

 (Brachio)iis, Koteus, Mastigocercu, Stejihanoj^s, etc.); third, Hydatinidae (7?y(?a<«;ia, 

 Monocerca, JSTotonimata, Diglena, etc.). We shall give au account of a typical species 

 of the second and third family each, 

 having already described the type of 

 the first. 



In the Beachionidjs 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. IToteus 

 qiiadricornis 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. 



Hgdatina senta is a classical ani- 

 mal, liecause it was principally on this 

 species that the illustrious Ehrenberg 

 studied the anatomy of this group of 

 animalcules. The broad bodj- has only 

 a very short foot-stalk, which is forked 

 behind. The mouth is ai-med with 

 two jaws and many teeth. There are 

 no eye-specks whatsoever. The cuticle 

 is delicate and soft. Hydatina illus- 

 trates the extreme ra]iidity with which 

 the development of the young is ac- 

 complished. The egg is extruded 

 witliin a few hours after the rudiment 

 (if it becomes distinctly visible. Within twelve hours more the shell bursts and the 

 }oung animal comes forth. In Rotifer and several other genera the young are extruded 

 ali\e, incubation being completed within the parental body. In Brachionus still a third 

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

 of the body, until the yo\mg are set free. Ehrenberg has made the astounding estimate 

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

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

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



Fig. 190. — Nolens, quadriconiii 



