428 



THE CIRRIPEDS, TUNICATES AND ECHINODERMS. 



and caudal regions the asymmetry is less apparent. (Fig. 294.) As the right 

 half of the oral surface of the thorax takes on a circular form, its haemal surface 

 divides into five thickened, tubercular, and calcareous plates, each plate corre- 

 sponding to one of the thoracic appendages and representing the right half of a tho- 

 racic tergite. (Figs. 290, 292.) Each of the five plates, or half tergites, becomes 

 the aboral surface of a starfish arm. The cephalic and caudal ends of the thorax 

 finally unite, forming the five-rayed body of the new animal. (Figs. 294, 292, G.) 

 The mouth, m, is drawn into the center of the neural surface, and is surrounded 

 by the remnants (right half ?) of the nerve cord, while the five pedal nerves form 

 the radiating nerves of the arms, and the thoracic coelomic chambers form the 

 circular and radiating water vascular canals. The anus and madrepore are 

 crowded toward the center of the haemal surface. 



ab cl. 



Fig 294. — Diagrams illustrating the probable origin of the echinoderms from asymmetrical, cirriped-like larvK. 

 The larval organs are seen from the neural surface, in mercator projection. E, Hypothetical symmetrical larva; F, 

 hypothectia asymmetrical larva; G, the radially symmetrical echinoderm. 



When the circle is completed, the head of the larva is either gradually drawn 

 into the disc and absorbed, or according to Corens and Danielson, it is amputated, 

 and for a short time may lead a separate existence after its separation from the 

 posterior part of the body, thus recalling the amputation thatoccurs in Sacculina 

 near the close of its metamorphosis, except that here it is the head alone that 

 survives; the abdomen dies. 



The Vestibule, or Peribranchial Chamber. — In the starfish, during the forma- 

 tion of the appendages, the neural surface of the thoracic region is depressed and 

 partly enclosed by the thoracic folds, forming a rudimentary atrial or vestibular 

 chamber into which the primary tentacles project like the appendages of an ar- 

 thropod into the peribranchial chamber. In Echinus and Antedon, the right peri- 

 branchial chamber deepens at a very early period and forms a closed vestibule, 

 from the floor of which the five primary tentacles develop in the usual way. 

 (Fig. 295, D.) Finally the membranous roof of the vestibule ruptures, allowing 

 the appendages to protrude (Fig. 295, G), as they do in a cirriped, or in a 

 polyzoan, (Fig. 301.) 



Asymmetry. — The morphology of the echinoderms is dominated by a 

 strongly marked asymmetry. While asymmetry of originally symmetrical 

 animals is no doubt common enough as an embryonic abnormality, it is rarely 



