264 THE ANATOMY OF INVERTEBRATED ANIMALS. 



The Cumacea take an intermediate position between the 

 Podophthalmia and the Ectriophthalmia on the one hand, 

 and the Phyllopoda (N'ebalia) on the other. They thus serve 

 to connect the Malacostraca with the Entomostraca. 



THE PODOPHTHALMIA. It will be convenient to commence 

 the study of the Malacostraca with the Podophthalmia ; and 

 as excellent examples of this division of convenient size are 

 readily obtainable in the fresh-water Crayfish (Astacus fluvi- 

 atilis) and the Lobster (Homarus vulgaris), and as they fur- 

 nish a very intelligible guide to the general plan of structure 

 of the higher Arthropoda, the organization of Astacus will 

 be described at length. With some unimportant modifi- 

 cations, what is said about it will be found to apply to the 

 Lobster. 



The upper and anterior portion of the dense and more or 

 less calcified exoskeleton which covers the body of Astacus^ 

 has the form of a large, expanded, shield-like plate, the cara- 

 pace, produced into a strong frontal spine between the eyes, 

 and bent down at the sides, so as to reach the bases of the 

 legs. The posterior division of the body, on the other hand, 

 presents a very different aspect, being divided into a series of 

 distinct movable somites. This is called the abdomen ; while 

 the anterior division, covered by the carapace, corresponds 

 with the head and thorax of other Arthropoda, and receives 

 the name of cephalo-thorax. 



On turning to the ventral surface of the Crayfish, a great 

 number of limbs or appendages, twenty pairs in all, are seen 

 to ba attached to the cephalo-thorax and abdomen, six pairs 

 belonging to the latter and fourteen pairs to the former re- 

 gion of the body. 



The six pairs of abdominal appendages are commonly 

 known as the "false" or "swimming" feet ; and it will be 

 observed that the\' are attached to the six anterior segments 

 of the abdomen only, the seventh being unprovided with any 

 such organs. Of the fourteen pairs of cephalo-thoracic ap- 

 pendages, the five posterior are called the " ambulatory " legs, 

 being the organs by which the Crayfish is enabled to walk. 

 Strictly speaking, however, the anterior of the five pairs is 

 not more ambulatory than prehensile, being so modified as to 

 constitute the great claws, or " chelas." 



Of the six next pairs of appendages, passing from behind 

 forward, five are not at first sight apparent, the posterior 

 pair, which are applied over the mouth and cover the others, 



