8 ECHINODERMA GENERAL DESCRIPTION 



forming the "hydropore"and the "pore canal"; while, in no connec- 

 tion with this, a groove arises along the hinder wall of the anterior 

 coelom, and develops into a canal connecting the left hydrocoel 

 with the anterior coelom, and called "stone canal," because its homo- 

 logue in recent adult Echinoderms develops spicules in its walls 

 (Fig. VI. 11). MacBride (1896) has observed larvae of Asterina 

 'gibbosa, in which there were a hydropore and stone canal on the 

 right, and some in which both right and left pores were present. 

 The latter arrangement occurs also temporarily in the Bipinnaria 

 of Asterias (Field, 1892, Fig. VI. 10), and is that which we suppose 

 to have obtained in the Dipleurula. 



On the ventral side, at the anterior end of the body, a mouth 

 is produced by invagination, and leads into the remaining part of 

 the archenteron, which becomes modified into a larval stomach and 

 a short rectum curved ventralwards and opening at the blastopore. 

 The part of the larva in front of the mouth is called " the preoral 

 lobe," and a portion of it becomes a sense organ, usually ciliated, 

 with a development of nerve tissue (Fig. VI. 9, 10; Fig. I.). 



It must not be supposed that a Dipleurula larva of this simple 

 type actually exists. In each class it presents some modification, 

 the outward appearances of which have already been described. 

 Moreover, the internal structures vary in the order of their 

 development and in persistence. Enough is common to the 

 various types to show that the Dipleurula larva is no phantasm, and 

 to suggest very strongly that it represents an ancestral Dipleurula 

 stage, differing but slightly if at all from the ancestral Tornaria, 

 and being one of the lowest of all animals with a coelom. The 

 hydrocoels and their indirect exterior openings have been compared, 

 perhaps not very judiciously, with the excretory nephridia of 

 higher Coelomata. The possible connection of Tornaria with the 

 ancestral Chordata gives additional interest to the resemblance 

 between stereom formation and bone formation (see p. 29), and 

 to the invagination of a primitively superficial nervous system in 

 the two groups. 



Between adult Echinoderms and other groups of the Animal 

 Kingdom no comparisons are possible. From this stage onward 

 the Echinoderm follows a path of its own. By a remarkable 

 metamorphosis, varying in its details but presenting some common 

 features in the different classes, the almost bilaterally symmetric 

 larva is transformed into the almost radially symmetric adult. 

 This metamorphosis undoubtedly represents the changes that took 

 place in the early history of the classes; and the extraordinary 

 difficulties of interpretation are due to the enormous compression 

 of that history, the elimination in some cases of unnecessary 

 stages, and the unequal acceleration of others. The clue is offered 

 by the older fossils, which, as explained under Cystidea, forcibly 



