NO. II ECHINODERMS AS ABERRANT ARTHROPODS CLARK 5 



The larval nerves disappear; but some time after the torsion of the 

 animal is completed a new nerve mass forms in exactly the place 

 that the preceding nerve mass occupied, now become the point just 

 above the top of the column. It is thus natural to assume that the 

 central mass of the apical nervous system in the adult crinoid, ap- 

 pearing in the region previously occupied by the anterior nerve mass 

 of the larval crinoid, corresponds to it, and therefore that it repre- 

 sents the anterior nerve mass in other invertebrates. But as a result 

 of the torsion which. the animal has undergone the right ccelomic sac 

 has become extended anteriorly and, following the enteric wall, has 

 reached over onto the left side ; from its anterior end it gives ofif five 

 anteriorly directed diverticula which at a later stage become cut ofif 

 and give rise to the chambered organ about which the central nerve 

 mass forms a close and almost complete investment. 



The central nerve mass gives of¥ five stout cords which immediately 

 branch, the two branches from each being connected by one or two 

 commissures, and then joining the similar branches from the nerve 

 trunks on either side to form the radial nerves of the division series 

 and arms. In each segment of the arms and pinnules, the nerve cord 

 ■gives off from a ganglionic swelling four branches, two dorsolateral 

 and two ventrolateral. In addition to these nerve cords, which may 

 be either single or double, and are in some types represented by two 

 widely separated parallel cords, each cirrus in its central canal con- 

 tains a prolongation from the chambered organ ensheathed in nerve 

 fibers continuous with those of the central nervous mass, and five 

 similar prolongations from the chambered organ and the central 

 nerve mass extend downward into the stem. 



THE ECHINODERM CCELOME 



Much has been made of the fact that in the echinoderms the 

 coelome is enterocoelic in origin, as in the Brachiopoda, Chsetognatha, 

 Chordata, and probably the Phoronida, while in the developing mol- 

 luscs, annelids and arthropods the coelome is not enterocoelic in origin. 



Sedgwick notes that in all the chordates except the tunicates the 

 coelome in its first state in the embryo shows more or less marked 

 traces of three divisions, the anterior or proboscis coelome. which in 

 the Vertebrata and Enteropneusta is single and in Amphioxus double ; 

 the collar or middle coelome, which is always double, and the trunk 

 coelome, which is double and which in the Vertebrata and in Am- 

 phioxus becomes metamerically segmented. In the echinoderms 

 there seem to be indications, at least, of a similar tripartite division; 



