376 Dr. O. Hamann on the Phylogeny 



between the sutured lines or Semites of the Spatangida; only 

 in these the nerve-fibre layer, which is epithelial in position, 

 is more strongly developed generally in the whole of the 

 dorsal epithelium, but especially in that of these s.utural lines. 

 Nerve-terminations are observed in the amhulacral feet, 

 especially in the peculiar pencil-like foot of the Spatangida. 

 The complex structure which occurs in the sucking-plate of 

 the foot of a regular Sea-urchin can only be described by 

 reference to the figures. 



In the epithelium, the epidermis, which covers all the ex- 

 ternal organs, nerve-fibres occur everywhere. They are all 

 epithelial in position, or only partially so ; in the latter case 

 they run subepithelially in the layer of connective substance, 

 the cutis. The body-wall of a Sea-urchin is composed, as is 

 well known, of the outer epithelium and the cutis with the 

 calcareous plates or separate calcareous bodies, as, for ex- 

 ample, in the buccal disk, or also at the vertical pole (in 

 Ceoitrostephanus longispinus). In the body-wall, and indeed 

 in the middle of the paired, so-called ambulacral plates, run 

 five longitudinal canals. They commence at the vertical pole 

 beneath the five intergenital (ocellar) plates, and run to the 

 lantern, the masticatory apparatus. They are schizocoele- 

 structures, longitudinal canals, in the connective layer. Into 

 them have been shifted the five ambulacral (or radial) nerve- 

 trunks, which in the Starfishes are still situated in the ecto- 

 derm. These nerve-trunks terminate on the one hand in the 

 intergenital plates, on the other they pass into the Jantern and 

 form a nerve-ring which, on one side, is enveloped by a con- 

 tinuation of the longitudinal canals. In and upon the inter- 

 genital plates there is a rudimentary tentacle without any 

 visual spots. The nerve-trunks consist of very fine nerve- 

 fibres and ganglion-cells and of a cellular coat which is in 

 part composed of supporting cells. This epithelium is to be 

 regarded as homologous with the epithelium of the ambu- 

 lacra] grooves of the Asterida, not only the nervous mass 

 itself, but the whole epithelium, having come to be situated 

 in the mesoderm, as in the Holothurij^. 



From the nerve- or central ring nerve-cords are given off 

 to the oesophagus, and these may be traced throughout the 

 whole course of the intestinal tract. Parallel to the ambu- 

 lacral nerve-trunks run the five amhulacral water-vessels ; 

 they terminate csecally in the intergenital plates, while at 

 the masticatory apparatus they ascend upon its outer surface 

 and enter into the water-vascular ring, which lies upon the 

 surface of the masticatory apparatus (the lantern) and sur- 

 rounds the oesophagus. From this water-vascular ring the 



