PHYLLOPODA AND CLADOCERA. 



215 



FOURTH LEGION. BR ANCHIO POD A. 



ORDER VIIL PHYLLOPODA (LEAF-FOOTED). 



The BRANCHIOPODA, or Gill-footed Crustacea, form the first division 01 the ENTOMOPTKACA, or 

 " Shelled Insects," so called because most of its members are more or less entirely invested in a 

 shelly envelope. 



They are all aquatic, the greater part having a shell composed of two parts or valves, in which 

 they are more or less completely enclosed, or in the form of a buckler, which envelops a large part 

 of the animal. Their gills are attached to their feet, or to their jaw-feet. Like the higher class of 

 Crustacea, they moult their shell and skin frequently. 



Of the shield-bearing form of PHYLLOPODA (Fig. 38), the fresh-water Apiis may serve as a good 

 example. The eyes are placed on the dorsal surface of the carapace, and are nearly united. The antennae 

 are short and simple ; the first pair of feet are very long and branching ; these are followed by about 

 sixty pairs of branchial feet. The thorax and 

 abdomen are nearly cylindrical, and are com- 

 posed of about thirty articulations, terminated 

 by two long, many -jointed tail-spines. 



Apus affords a good example of a form in 

 which the mere vegetative repetition of parts is 

 carried to an extreme distance beyond the 

 normal number of body-rings so characteristic 

 of the Crustacean class. Probably Apus has 

 more articulations to its appendages and body 

 than any other Crustacean. Schaffer tabulated 

 them, and found they numbered 1,802,604 ; La- 

 treille puts them down at not less than 2,000,000. 



In Nebalia, the marine type, the head- 

 shield is more arched, covering the body as in a 

 bivalved shell. The eyes are pedunculated, and 

 placed beneath the carapace. The number of 

 segments is not excessive. 



Otho Fabricius says that "the female carries her eggs beneath the thorax during the whole 

 winter ; these begin to hatch in April, and appear in May, when they are very lively, and adhere to 

 the mother. The adult is not very active. On our coast they are found under stones, lying on mud 

 amongst hollows of rocks." 



The genus Estheria deserves to be especially mentioned on account of its wide distribtition at the 

 present day, and also because it has a very long past geological history. Its oval, bivalved shell has 

 often been mistaken for that of a mollusc. 



In Cheirocephalus and Artemia the shelly shield is altogether wanting, and their elegant move- 

 ments in the water can be freely observed. The former inhabits fresh water ; the latter is marine. 



ORDER IX. CLADOCERA. 



In the CLADOCERA* the body, save the head, which is projecting, is entirely enclosed within a 

 carapace, formed by the two valves of the shell. The eye is single and very large ; the four to six 

 pairs of feet are branchiform, the two large pairs of antennse serving as organs of locomotion. 

 Of this order, the Common Daphnia pulex (Fig. 39) of fresh waters is the best example we can 

 take, not only on account of its abundance, but also because it has formed the subject of numerous 

 memoirs by Professor Leydig and others. So plentiful are they in some ponds as to impart a blood-red 

 hue to the water frequented by them. In order to apprehend the wonderful fecundity of this 

 and allied genera, it is necessary to realise that a Daphnia, under favourable circumstances of 

 temperature, may have three broods a month, or even a greater number, some of the larger species 

 having as many as forty or fifty eggs at one brood ! 



'' At particular seasons the Dapknice may be found with a dark opaque substance on the back of 

 * Greek, klados, a branch, and keras, a horn ; hence branching-horned, in allusion to their antenna. 



Fig. 38. PHYLLOPODA. 



1. Sebalialnves (shell partly removed); 2, Esther ia, sp. ; S,Lepidunis angassi 

 . dorsal aspect, ft. ventral aspect; 4. Larva of Apits cancriformis ; J>, 

 Branchiims (Cheiroeei>halus) Btagnalis a, adult female, &, c, larva; ; 6, 



Larva of AdemM salina. 



