22 Prof. J. Miiller on the Structure of the Echinoderms. 



Laganum Bonanni has only two marginal chambers common 

 to the dorsal and ventral walls. The pore-arese on the outer 

 surface of the shell resemble those of the Clypeasters ; only every 

 ambulacral plate has its peculiar pore- area, narrow poreless bands 

 intervening between them. On the internal surface of the 

 shell every ambulacral plate presents an especial pore-area, whose 

 pores are not disposed in many transverse series ; in the mar- 

 ginal chambers we meet with many parallel series of pores. The 

 dorsal surface of the shell also presents certain peculiarities; 

 small pores are perceptible upon the internal surface of the pe- 

 taloid ambulacra, which spread out upon the peripheral portion 

 of the back ; and the interambulacral arese also, between these, 

 contain similar fine pores upon those portions of the interambu- 

 lacral plates which are contiguous to the large branchial double 

 pores, where, in other cases, locomotive pores are never found : 

 specimens preserved in spirit should be examined with regard to 

 this point. 



I now pass to the Clypeasterida with ventral ipore-fascia. The 

 internal area of the petaloid ambulacra presents the same rela- 

 tion in Echinarachnius as in the Clypeasters and Arachnoides, 

 and it would therefore seem, at first, as if this character were uni- 

 versal in the entire family of Clypeasteridtp.. However, in spirit 

 specimens of Mellita quinquepora and hexapora, and Lobophora 

 hifissa, for which I am indebted to MM. Krantz, Eschricht, and 

 Steenstrup, I have been unable to find either the small ampullae or 

 feet on the lateral branches of the ambulacral vessel in the peta- 

 loid ambulacrum. The peripheral dorsal part of the ambulacra 

 is always present. Not far distant from the edge, the ambulacral 

 vessel of Mellita quinquepora forms a pennate ramification on 

 the imperforate anterior radius. These curved parallel branches 

 lie in canals of the shell, which are here and there connected 

 with other sinuses of the shell. 1 have also persuaded myself of 

 the presence of feet upon the peripheral part of the dorsal shell 

 of Mellita. 



In Echinarachnius parma the ambulacral galleries are absent, 

 and we find only transversely jierforated partitions and rafters 

 between the dorsal and ventral walls at the periphery of the disc, 

 constituting imperfect marginal chambers. On the ventral part 

 of the ambulacrum of Echinarachnius, we observe from the mouth 

 to the peripheral galleries, on the inner surface of the shell, 

 many short pennate branches with appended ampullae. It is 

 from these therefore that the pores of the median pore-fasciae 

 must be supplied. In correspondence with the lateral branches 

 of the fascia externally, there run internally long pennate 

 branches of the ambulacral vessel provided with many ampullae. 

 The further ramifications take place in the peripheral galleries. 



