X PHYLUM ARTHROPODA 229 



size. The ova, when laid, are fastened to the setae 

 on the pleopods of the female by the sticky secretion 

 of glands occurring both on those appendages and on the 

 segments themselves : they are fertilised immediately after 

 laying, the male depositing spermatophores on the ventral 

 surface of the female's body just before oviposition. 



The Lobsters, Rock-lobsters, Shrimps, Prawns, Crabs, and 

 Hermit-Crabs all resemble the Crayfish in the number and 

 disposition of the segments, the presence of a carapace 

 covering both head and thorax, the general structure 

 and arrangement of the appendages, and the essential 

 features of the internal anatomy. The Crabs and the 

 Hermit-Crabs differ from the other forms mentioned mainly 

 in the abdomen being reduced. In the Crabs (Fig. 122) this 

 region is extremely small, has the appendages only feebly 

 developed, and is permanently flexed on the sternal surface 

 of the cephalothorax, so that it is completely concealed from 

 view when the animal is looked at from above. In the 

 Hermit-Crabs (Fig. 123) the abdomen with its appendages is 

 imperfectly developed and not completely enclosed in a 

 hard exoskeleton, this region being sheltered in the shell of 

 a Whelk or other univalve Mollusc, which the Hermit-Crab 

 drags about with it. 



The Crustaceans enumerated above, together with the 

 Sand-hoppers, Wood-lice and their allies, and a large 

 number of others, form one of two sub-classes into which 

 the class Crustacea is divided the sub-class Malacostraca. 

 The Malacostraca are highly organised Crustacea, usually 

 of considerable size, and nearly all have a thorax of eight and 

 an abdomen of seven segments. The appendages are highly 

 differentiated. There is a gastric mill, and the renal organs 

 are in the form of antennary glands. 



