134 PRIMITIVE FORMS OF LIFE 



surface is more developed than their ventral concave surface, 

 so that they might be regarded as curved like a C. Bands of 

 vibratile cilia, originally arranged round them in a girdle, as 

 in the larvae of the Synaptidae and the Crinoids, but deformed 

 by the excessive growth of certain parts of the embryo, 

 particularly the dorsal surface, divide the body into five 

 segments, whence the term pentatrochal applied to these larvae. 

 If the organism remained in this condition there would be no 

 hesitation about including it among the Annelid Worms. 

 Soon, however, calcareous spicules appear in its tissues, and at 

 the same time the internal organs become dissymmetrical ; 

 those of the side which develops most rapidly present the 

 characteristic feature of rolling up in a spiral. Shortly after 

 the appearance of calcareous spicules in the tissues, the young 

 organism, grown in weight, ceases to swim and falls to the sea- 

 bottom, and there its posture is steadily modified, the left and 

 right sides becoming respectively the ventral and dorsal 

 aspect of the adult animal. If we take these constant develop- 

 mental phases of the Echinoderms as patrogonic in origin — 

 and to do otherwise is to deprive embryogeny of all significance 

 ■ — the phylogenetic history of these animals appears to be as 

 follows. Their ancestral form was that of a short Annelid Worm, 

 reduced to rive segments, whose body, merely because of its 

 muscular tonus, became curved into a C like the majority of 

 segmented animals in their fixed state. 1 This worm originally 

 a swimmer, secreted lime, which was deposited in the form of 

 spicules in its tissues, thus gradually increasing its weight. 

 It finally fell to the bottom, and, having become rigid and 

 incapable of recovering its position, owing to this development 

 of spicules, so, because of its curvature, it remained lying on 

 its side. Thenceforward it became dissymmetrical, like the 

 larvae of Amphioxus and the Tunicates, and the Pleuronectoid 

 Fishes, which have changed similarly. But an animal lying on 

 its side is unfavourably situated for securing nourishment, for 

 it is particularly on the floor of the sea that the food it requires 

 must be sought. It is therefore forced to bring its mouth to the 

 floor, using for this purpose all the muscles at its disposal. 

 Guided by what Lamarck called the sentiment of need or of 



1 Woodlice and other Isopod or Amphipod crustaceans ; the larvae of 

 Cockchafers among the Arthropods ; Aphrodite and other crawling Annelid 

 Worms with short bodies and ventral surfaces with strong muscles. 



