Oral. 



340 PHYLUM ARTHROPODA 



Apus is over an inch in length, a giant among Entomostraca. It has 

 an almost world-wide distribution. The appendages are very 

 numerous and mostly leaf-like. They may be regarded as 

 representing a primitive type of Crustacean limb. Professor 

 Ray Lankester enumerates them as follows : — 

 (1. Antenna. 

 Pre-oral. -! 2. Second antenna. (This is sometimes absent, and 



y apparently always in certain species.) 



3. Mandible. 



4. Maxilla. 



5. Maxillipede. 



f6. First thoracic foot (leg-like). 

 Thoracic | 7-16. Other ten thoracic feet (swimmers). 



(Pregenital). 1 The i6th in the female carries an egg sac or broad- 

 ly chamber. There are eleven thoracic rings on the body. 

 Abdominal f 17-68. Fifty-two abdominal feet, to which there corre- 

 ( Post -genital). \^ spond only seventeen rings on the body. 



The large dorsal shield is not attached to the segments behind the 

 one bearing the maxillipedes. Many of the thin limbs doubtless 

 function as gills. The genital apertures are on the sixteenth 

 appendages. The anus is on the last segment of the body. 

 There is a pair of ventral ganglia to each pair of limbs ; the ventral 

 nerve-cords are widely apart ; and the cephalic ganglion is 

 remarkably isolated. There is periodic parthenogenesis. 

 (6) Cladocera. Small laterally compressed " water-fleas," with few 

 and somewhat indistinct segments. The shell is usually bivalved, 

 and the head often projects freely from it. The second antennae 

 are large, two-branched, swimming appendages, and there are 

 4-6 pairs of other thoracic appendages, which, vibrating very 

 rapidly and provided with rows of setae, serve both as a pump 

 and as a sieve on which all the food-particles in the water 

 pumped through are retained. The heart is a little sac with 

 one pair of openings. An excretory organ (the shell or maxillary 

 gland) opens in the region of the second maxillae. It is the 

 Entomostracan equivalent of the antennary green gland of 

 Malacostraca. The males are usually smaller and much rarer 

 than the females. The latter have a brood-chamber between 

 the shell and the back. Within this many broods are hatched 

 throughout the summer. Periodic parthenogenesis (of the 

 "summer ova") is very common. "Winter eggs," which 

 require fertilisation, are set adrift in a part of the shell modified 

 to form a protective cradle or ephippium. 



Daphnia, Moina, Sida, Polyphemus, Leptodora, and many 

 other " water-fleas," are extraordinarily abundant in fresh 

 water, and form part of the food of many fishes. A few 

 occur in brackish and salt water. 

 In Daphnia the appendages are : — antennules, antennae, 

 mandibles, first maxillae, second maxillae (disappearing in the 

 larva), and five thoracic limbs. The abdomen is turned 

 downwards and forwards, and shows three segments and a 

 telson. 



