124 PRIMITIVE FORMS OF LIFE 



vibratile cilia together, and cause them to disappear; in the 

 other, the vibratile cilia, arranged like fleece or in a ring, per- 

 sisted, and formed the primitive organs of locomotion. Failing 

 them, the merids of the first type were obliged to propel themselves 

 by means of lateral buds moved by muscles and provided with 

 rigid filaments, which developed into feet. From this type 

 arose the long series of Arthropod forms. The ciliated type 

 produced the series of Annelid Worms, which share with the 

 Arthropods all these characters of organization resulting from 

 their powers of locomotion ; Cuvier had already united the 

 two in order to form his group of Articulata. However, the 

 two series differ entirely in all those qualities entailed by the 

 absence of vibratile cilia ; they have evolved separately on 

 parallel lines, with no bond of relationship between them. 



Embryogeny gives us some idea of what these primitive 

 merids may have been. All the Crustaceans of the large 

 sub-class Entomostraca, however complicated they may be, 

 are born in the form of small organisms, called nauplii, with 

 only three or even two pairs of appendages surrounding the 

 mouth. These serve, primarily, as swimming organs, but at the 

 same time they hold prey by means of hooks borne by their 

 proximal joint. These appendages, after having been employed 

 simultaneously as legs at their free end, and as jaws at their 

 base, develop into the two pairs of antennae and the mandibules 

 of the adult. Various species of higher Crustaceans have 

 continued to hatch out at the nauplius stage, notably the 

 large edible prawns 1 found along the Mediterranean coast. 

 The embryogeny of certain fossil Trilobites of the Primary 

 Epoch has also been reconstructed. The species of Sao, for 

 instance, were born with only three segments, the others 

 being formed successively in front of the telson. 



At birth the free marine Annelid Worms whose bodies are 

 not divided into distinct regions, and which have been called 

 Annelida Errantia, appear with a still simpler form, which, 

 strictly speaking, represents only the first segment of the adult 

 animal and the telson ; this is the trochosphere, an ovid body, 

 barred with two rings of vibratile cilia between which lies the 

 mouth. 



Starting from these initial stages we can follow, in the two 

 series of Arthropods and Vermes, every step by which an 



1 Penaeus scaramota. 



