Tate : The Organization of " Daphnia Pulex." 133 



arise near the articulations of the body segments to which they 

 belong, each foot being provided with an extensor and flexor. Each 

 arm has four principal muscles arising at various points along the 

 dorsal line of the cephalic shield, giving off fibres to the several 

 joints. A long narrow muscle, arising in front of the cseca, passes to 

 the base of the labrum and deflects it, while a short muscle from the 

 base of the beak, flexes the distal end of the labrum upon itself. The 

 eye, which is capable of rapid and incessant semi-rotary movements, 

 is governed by a plexus of muscular fibres, branching and anasto- 

 mosing in a manner it would be difficult to describe. The circular 

 anal sphincter is the part of the embryo first to exhibit that vitality 

 which in the adult it is the last to forfeit. Its contractions and 

 dilatations are often continued for fifteen or twenty minutes after all 

 other parts of the organism appear to be dead. 



Throughout Artliropoda the circulation is more or less extremely 

 lacunar as in Daphnia. The apparent complexity which on first 

 inspection their circulation presents is due to the fact that three dis- 

 tinct vascular systems are seen at once, and it is only by careful 

 focussing that their separate courses can be clearly defined. These 

 are the systemic, the branchial, and the tegumentary. The simple 

 globular heart is suspended within a square thoracic cavity above the 

 stomach, and is attached by the centre of its anterior aspect to the 

 first (cephalo-thoracic) articulation. Close below this attachment lies 

 an orifice out of which the large colorless corpuscles may be seen to 

 emerge after each contraction. The pulsations are extremely rapid, 

 ranging from 150 to 200 per minute when the animal is free, but they 

 may be rapidly reduced at the will of the investigator down to 10 or 

 even 5 per minute. The blood corpuscles on quitting the heart, pro- 

 ceed by a canal along the dorsal surface of the stomach as far as the 

 coeca, where they divide to the right and left, floating freely in the 

 perivisceral cavity, bathing the brain, gullet, labrum, mandibles, the 

 lateral and ventral surfaces of the alimentary canal backwards to its 

 termination at the rectum ; round which they may be seen rapidly 

 curving, and returning, by another canal, along the dorsal surface of 

 the intestine and stomach ; emptying its contents into the thoracic 

 cavity behind and below the heart. The corpuscles then furtively 

 find their way to one or other of two lateral slits situated on the 

 upper surface of the heart. Such is the course of the systemic cir- 

 culation : — forwards, from the heart to the brain on the dorsal aspect ; 

 backwards, from the mouth to the rectum ventrally ; and forwards 



