372 Dr. O. Hamann on the Fhylogeny 



covered by GreefF In the connective substance of the cutis of 

 the dorsal wall. These cavities are connected with the peri- 

 hseraal spaces and the so-called tubular canal. 



Taking all this into consideration we have in the Asterida 

 a series of schizocoele-structures to which the five or more 

 ventral radial canals (perihgemal spaces) belong, and, further^ 

 the blood-lacunse running through septa, likewise schizocoele- 

 cavities. 



What do we find of these two systems of cavities in the 

 Echinida ? 



In the regular Echinida we find the five longitudinal 

 canals, in which the five ambulacral or radial nerve-trunks 

 have come to lie. Further, we find a cavity enveloping 

 the annular nerve at one of its surfaces, a homologue of 

 the annular perih^mal cavity in the Asterida. Besides 

 these we have to note an anal annular schizocoele-cavity, 

 with cavities which run to the sexual organs. These are the 

 sole remains of the great dorsal canal-system of the Asterida ; 

 in their wall, i. e. in the wall of the anal schizocoele-cavity 

 and partly projecting into it, lies the anal ring of blood- 

 lacunse, and, in the cavities running to the sexual organs, 

 the blood-lacunae. Consequently in the dorsal part of the 

 Echinida the same conditions exist as in the Asterida. Here 

 also we may speak of perihsemal cavities. The different 

 character of the ventral surface is to be explained by the 

 formation of the masticatory apparatus and by displacement 

 of the five radial nerve-trunks, which have deserted their 

 epithelial position and moved into the schizocoele-cavities. 

 The septa with the blood-lacunse (in the longitudinal canals), 

 and therefore the true radial blood-lacunse of the Asterida, 

 have disappeared. But, on the other hand, a perioesophageal 

 ring of blood-lacunse has become developed upon the lantern, 

 and from this the blood-lacunse run, as in the Asterida, to the 

 gland, intestine, &c. 



In the Spatangida, which are quite certainly to be derived 

 from the Echinida, these conditions are as follows : — The 

 masticatory apparatus has disappeared, and with it the peri- 

 oesophageal ring of blood-lacunas situated upon it. In the 

 five longitudinal canals (perihaimal canals) which open into 

 an annular canal situated around the oesophagus lie the 

 (radial) ambulacral nerve-trunks and the circumoral nerve- 

 ring as in the regular forms. But the blood-lacunae (dorsal 

 and ventral) of the oesophagus open into this annular schizo- 

 coele-canal. This, consequently, in the Spatangidse is to be 

 regarded as a blood-lacunar ring, and the five longitudinal 

 canals given off from it as the five ambulacral blood-lacunse. 



