THE ECHINODERMS. CCELOM. APPENDAGES. EXCRETORY ORGANS. 427 



The thoracic somites, as in the arthropods, remain relatively small and without 

 lateral plates, and do not expand laterally onto the haemal surface. 



The more posterior division of the mesoderm, ab.cl., probably represents 

 several abdominal somites and lateral plates which have combined to form the 

 general body cavity, or ccelom. Like the corresponding structure in the arach- 

 nids (Fig. 138), it extends rapidly in a cephalic, haemal, and caudal direction, till it 

 meets the opposite ccelom. (Fig. 294, E.F.) The mesentery formed by this 

 union naturally lies to one side of the median haemal line owing to the unequal 

 growth of the two chambers. 



Thus the principal difference between the mesoderm in a young echinoderm, 

 as shown in mercator projection (Fig. 294, F.), and that of an arachnid (Fig. 

 138), lies in the asymmetrical development of the mesoderm and in the absence 

 of segmentation in the abdominal ccelom. 



Thoracic Appendages. Sometime after the five chambers of the hydroccele 

 appear, finger-like outgrowths of the ectoderm are formed over the thoracic 

 somites. They are the five primary tentacles, or tube feet, which represent five 

 modified thoracic appendages, th.ap. An outgrowth of the underlying somite 

 grows into each appendage, in typical arthropod fashion, but instead of breaking 

 up into separate muscles for the appendage it remains permanently in the form 

 of a membranous diverticulum of the hydroccel, and becomes the distal end of 

 a radiating water vascular canal. Only the distal end of the original appendage 

 separates from the body as the primary tentacle; the remainder of the appendage, 

 however long it may eventually become, may be regarded as lying in the surface 

 ectoderm, developing on either side, as it increases in length, paired cirri that 

 become the double row of tube feet for each arm, and into each of which a pro- 

 longation of the water vascular canal extends. (Fig. 294, G.) 



Excretory Organs. A portion of the ccelom, probably belonging to the cephalic 

 division, undergoes a special modification. A narrow dorso-lateral outgrowth 

 arises from it that unites with an ectodermic infolding on the anterior aboral, or 

 haemal surface. From it develops the stone canal and the madreporite. (Fig. 

 294, ec.d.) The ectodermic opening places the hydroccele in communication with 

 the exterior, so that the organ has often been compared, in whole or in part, to 

 an annelid excretory organ or nephridium. It is, however, more like one of the 

 typical excretory organs of the head region of the arthropods (shell gland, green 

 gland, coxal gland) which consists of thin-walled ccelomic sacs, with a thick-walled 

 tubular outgrowth of varying length, united to a short duct, infolded from the 

 ectoderm. The haemal location of the external opening to the duct in the echino- 

 derms is no doubt due to the same causes that have carried the corresponding, 

 or at least adjacent, cephalic appendages to the haemal surface, not only in the 

 echinoderms, but in the entomostraca, cirripeds and tunicates. 



The Formation of the Disc. By the time the larva is attached, the asymmetry 

 of the thoracic region has become very pronounced, due to the fact that practically 

 all the growth is now taking place on the right side of the thorax. In the cephalic 



