214 ARTHROPOD A [CH. 



body is enclosed within the carapace. Around the mouth are a pair 

 of mandibles and two pairs of maxillae, and the thorax carries six 

 pairs of biramous many-jointed limbs beset with numerous hair- 

 like spines, the lashing of which kicks food particles towards the 

 mouth (Fig. 83). These limbs are slender and flexible and thus 

 differ from the corresponding limbs of Copepods. 



Like some Copepods the Cirripedes are without a heart, and 

 the existence of special respiratory organs is doubtful. Unlike 

 other Crustacea they are, as a rule, hermaphrodite, the male and 

 female reproductive organs being united in one individual. A few 

 species are parasitic, chiefly on other Crustacea, and these have 

 reached a very extreme stage of degeneration. 



Sub-class B. MALACOSTRACA. 



The second of the two large groups into which the Crustacea are 

 divided contains most of the more familiar forms, such as Crabs, 

 Lobsters, Shrimps, Wood-lice, etc. For the most part the Malac- 

 ostraca are larger than the Entomostraca and the number of their 

 segments is a fixed one. In all except the first order, Leptostraca, 

 which is really a connecting-link between true Malacostraca and 

 the lower forms, the number of segments is nineteen and there are 

 nineteen pairs of appendages. One of the most marked characters 

 in the Malacostraca is the differentiation of the trunk into two 

 distinct regions, the thorax and the abdomen. It is true, as is 

 mentioned above, that many authors speak of an abdomen in the 

 Entomostraca, but by this they mean the hindermost segments 

 which with a few exceptions are devoid of limbs. In any En- 

 tomostracan if we examine the series of limbs behind the jaws we 

 shall find that they constitute a continuous series without any 

 sudden change in their character. In a Malacostracan, on the other 

 hand, we find an abrupt change at one point in the character of the 

 limbs. The hinder limbs or swimmerets (pleopods) are markedly 

 different from the front limbs, for whereas in the swimmeret both 

 forks of the limb, endopodite and exopodite, are equally developed, 

 in the last five pairs of limbs of the thorax (peraeopods) the endopo- 

 dite is large and the exopodite is small or absent. It is this difference 

 in character which defines the thorax from the abdomen. 



Although the division between the head and thorax is not always 

 apparent, as a rule we may assign five segments to the head, eight 

 to the thorax and six to the abdomen, which ends in an unseg- 

 mented flap called the telson. The reason for this want of a 

 definite boundary between head and thorax in the Malacostraca is 



