214 NATURAL HISTORY OF BHOMPSTON POND. 



In Planorbis (fig. 46), another extremely common 

 Gasteropod, the foot is oval and rounded both in front and 

 behind. It is joined to the body by a long stalk, and bears 

 on the left both the respiratory and the generative orifices. 

 The shell is flattened, quoit-shaped, and sufficiently trans- 

 parent to enable one to -watch the beating heart (a) with a 

 simple lens. The odontophore can be prepared for .examina- 

 tion as in the case of Limnsea (above described). The 

 central teeth will be found to bear two points, while the 

 lateral ones are three-pointed. The eggs (fig. 47), about five 

 in number, are laid on aquatic plants, or upon the glass side 

 of the micro-aquarium, where their development can be 

 conveniently watched. Long before the young leave the 

 egg capsules they can be watched constantly revolving within 

 the eggs ; they are developed in about twelve days. 



The interesting little crustacean known as a water-flea 

 [Simocephalus vetulus] (fig. 48) is present in vast numbers, 

 especially at the north end of the pond in the less stagnant 

 water, where this curious little animal is to be seen darting 

 through the water by rapidly repeated lashing movements 

 of its large biramous antennae (a). When at rest it suspends 

 itself in the water by attaching the endopodites of its antennae 

 to some fixed submerged support. The diminutive anten- 

 nules (p), each bearing a single lateral seta, terminate in a few 

 hairs, probably sensory in function. When microscopically 

 examined the body is seen to be enclosed by a ridged bi-valved 

 carapace (b), the cuticle of which is defined from that of the 

 head by a cervical suture. This water-flea differs from other 

 Daphniidae in having no posterior terminal spine of the 

 carapace. The ryhthmic pulsations of the heart (c) and the 

 constant motion of the thoracic (branchial) appendages may 

 be observed through the transparent carapace. On more 

 careful examination the blood, containing amosboid corpuscles, 

 can be followed as it leaves the heart, enclosed in its peri- 

 cardium, by an anterior opening and flows through definite 

 channels to the various parts of the body and carapace. In 

 the head the alimentary canal (e) (easily distinguished by the 



