A. Anatomy, Physiology, Embryology. V. Echinoidea. 19 J 



Duncan ( 4 ) gives the name Tag to that comparatively bare space on the test of 

 an Urchin which is situated on the aboral side of the branchial slit. In Coelopleurus 

 Maillardi each tag bears three bunches of tissue which seem to act as respira- 

 tory organs by increasing the surface of the common derm. 



Munier-Chalmas mentions various fossil Urchins in which the water-pores 

 are not limited to the right anterior genital plate, but may occur on one or more 

 of the others, or even on all five plates. In Hemipnemtes water-pores exist not 

 only in two or four of the genitals, but only on the three anterior radials. When 

 only one madreporite is present, it is not to be considered as a case of concentra- 

 tion of the pores, but as due to the disappearance of the pores on the other genital 

 plates. These are sometimes very numerous, Echinoconus subrotundus having 366 

 water-pores on 4 plates. Many common genera have only two or three genital 

 openings, though described as having four. The first one to disappear is that of 

 the madreporite, and then that on the left anterior genital. 



Ayers finds the number and position of the Sphaeridia of an Urchin to be con- 

 stant each for individual species, except in young stages. In most forms they are 

 on the free surface of the test, but in some Clypeastroids, Cassidnlarians, and Spa- 

 tangoids, they occupy closed cavities in the coronal plates. The second or reti- 

 cular kind of calcareous tissue described by Loven is really a system of canals 

 which runs through the first or vitreous substance, and is a modification of the 

 canalicular spaces within the spines. It contains long branching nerve-cells with 

 occasional ganglionic enlargements which form a complete chain from the central 

 nervous network to the superficial epithelium. They pass out through the external 

 openings of the canals and end in knotlike enlargements between the epithelial 

 cells. The Sphaeridia are thus highly specialised sense organs, probably ser- 

 ving a combination of functions (e. g. taste and smell) as suggested by Loven. 



Haacke ( 1 , 2 ) has found tetra- and hexa-merous individuals of Amblypneustes, 

 besides other abnormal forms. 



The cousins Sarasin describe as compound eyes a number of blue spots on 

 the test of a Diadema, which are best developed in the iuterambulacral zones. 

 Each contains several pyramids of a strongly refractile cellular material, the inner 

 ends of which are surrounded by a sheath of deeply pigmented connective-tissue 

 cells, and rest on a plexus of nerve fibrils with interspersed ganglion cells. 

 Physiological experiments also indicate that this structure is an eye. The authors 

 further describe minute vessels extending through the general subepidermic 

 nerve-plexus, and perhaps opening externally by cuticular pores. 



Koehler has repeated his injections of the vascular system in Echini and 

 has verified those facts the truth of which had been doubted by Carpenter [see 

 Bericht for 18S3 I p 128, 130-132]. Carpenter ( 3 ) explains his reasons for 

 continuing to doubt the communication of the ovoid gland with the exterior 

 through the madreporite, and the union of the two radial vessels before joining 

 the oral ring as described by Koehler. 



Prouho (') finds that there is no intestinal siphon between the internal marginal 

 vessel and the first coil of the gut in Dorocidaris, but that the intestinal wall in 

 this position is marked by a groove with a modified epithelial lining, and he con- 

 siders this be a rudimentary siphon. The collateral vessel, connected with the 

 external marginal vessel of Echinus is also absent in Dorocidaris. 



Prouho ( 2 ) describes the Pluteus of Dorocidaris papillata and the microscopic 

 characters of its tissues. Each of the vasoperitoneal sacs is early divided into 

 two lobes, one applied to the gullet and the other extending along the stomach 



