THE DEVELOPMENT OF ORGANISMS 761 



climatic conditions, but it may merely illustrate a constitutional 

 variation. 



Very striking is the prolongation of the ante-natal embryonic 

 period in the case of the viviparous Peripatus, which, with allied 

 genera, represents an early divergence from the ancestral stock 

 leading on to centipedes, millipedes, and insects. The type is a 

 survivor from an ancient fauna, and has persisted partly in virtue 

 of its cryptozoic habits, and partly because of a prolonged gestation, 

 which lasts for about a year, longer even than the antenatal life of 

 a foal within the mare. The advantage is that the young Peripatus 

 is born fully formed and able to look after itself. 



The eggs of crabs, e.g. the edible Cancer pagunis or the shore 

 crab (Carcinus mcenas), develop under the shelter of the female's 

 tail, and give rise to Zoaea larvae which become pelagic. These 



Fig. 122. 



Zoaea Larva of a Crab. After Roule. Note the spine rising from the cephalo- 

 thorax. There are appendages as far back as the last pair of maxillipedes. 



feed, grow, and moult, and undergo metamorphosis which results 

 in Megalopa larvae. Whereas the Zoaea has appendages only as 

 far back as the third foot- jaw (maxillipede), and is marked by 

 a prominent spine on the cephalothorax, the megalopa has lost 

 this spine and has appendages as far back as those correspond- 

 ing to the last walking legs of an adult crab. Like the zoaea, 

 it carries its posterior body-region (abdomen) in a line with the 

 anterior body-region (cephalothorax), not tucked in as in the adult. 

 Now these megalopas cease to be free-swinging and sink to the floor 

 of the shallow sea. After growing and moulting, they change into 

 miniature but fully formed crabs which creep further shorewards or 

 to the shore itself. In this case there is a long succession of larval 

 stages, but the cycle is contrasted with that of Mayflies, inasmuch 

 as the fully formed crabs may live for many years. 



The eggs of lampreys (Petromyzon) develop rapidly and give rise 

 in 10-15 ds^ys to minute larvae. About a month later these are about 



