MONOGRAPH OF THE EXISTING CKINOIDS. 347 



incloses all the organs in the body cavity, and within this a connective tissue layer 

 of variable thickness in which cells with fibers may appear. There is no true endo- 

 thelium. Though in Heliometra glacialis the connective tissue cells bordering the 

 lumen may be flattened here and there, such cells never form the entire lining. 



The contents of the blood vessels consist of a finely granulated coagulable 

 fluid which rarely shows cells. It is difficult to demonstrate the existence of this 

 fluid in the connective tissue layer of the digestive tube, since the latter is always 

 but very slightly developed. 



Sea water has access to the various subdivisions of the body cavity through 

 the calyx pores, but is entirely excluded from the blood- vascular system ; the latter 

 has no connection whatever with the water-vascular system. 



The blood-vascular system of Heliometra, Isocrimts. and such species of the 

 Comasteridae as have been studied is exactly like that of Antedon. 



LABIAL PLEXUS AND SPONGY ORGAN. 



P. H. Carpenter termed that part of the blood vascular system, composed 

 of very small lacunae, which lies about the esophagus the labial plexus. Vogt and 

 Perrier described this labial plexus as spongy tissue with branched vessels. 



In Promacliocnnus kerguelensis, Heliometra glacialis, and Solanometra ant- 

 arctica Carpenter found that a portion of the labial plexus between the mouth 

 and the anal tube differs very considerably in structure from the rest of this organ. 

 The limits of this portion appeared so well defined, and it differed so much from 

 the remainder of the labial plexus, that Carpenter proposed to designate it by 

 the name of spongy organ. As described by Carpenter, the spongy organ lies 

 between the mouth and anus on the left or "eastern" side of the esophagus and 

 therefore is slightly nearer to the left posterior than to the right posterior ambu- 

 lacrum. In its most fully developed condition, only found between the mouth 

 and anus, the spongy organ is a somewhat egg-shaped mass, consisting almost 

 entirely of a delicate network of connective tissue with more or less open meshes. 

 The latter are rather wider in Promacliocnnus than in any other crinoid which 

 Carpenter examined. The trabeculse forming its outer portion are much more deli- 

 cate than those nearer the surface, and Carpenter was unable to find that they 

 possessed any epithelial covering. The surface of the organ is more compact, with 

 larger trabeculse and generaly smaller meshes which are lined with epithelial cells, 

 being, in fact, the ends of those blood vessels forming the labial plexus which are 

 connected with the spongy organ mostly, if not entirely, on its ventral side. 



Carpenter notes that the spongy organ of Heliometra glacialis is more com- 

 pact than that of Promachocrinus, but is similarly situated in the space left by 

 the incomplete adhesion of the visceral and parietal layers of the peritoneum, being 

 suspended in this space by threads of connective tissue. It is practically the direct 

 backward continuation of the labial plexus at the eastern angle of the mouth, 

 where it is much more largely developed than on the opposite side. The relatively 

 thick epithelial walls of the vessels gradually disappear as they enter the spongy 

 organ, while the latter in turn passes insensibly backwards into the plexus of 



