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



number of the lateral apertures in the galleries is exactly half 

 that of the large double pores. 



At the peripheral part of the dorsal ambulacra as far as their 

 edge, and over the whole ventral part of the ambulacra, the am- 

 bulacral vessels in Chjpeaster, Arachnoides and EchinarachniuSf 

 give off pennate branches to transverse series of pores, and are 

 again provided with a coiTesponding number of ampullae. In 

 Clypeaster Uangianus about 100 transverse series of pores may 

 be counted from the Inouth to the peripheral edge ; on the back, 

 from the edge to the pctaloid ambulacra, about 40 ; the number 

 in a transverse series increases from the mouth to the edge and 

 diminishes again, from the edge to the petaloid ambulacnun ; 

 towards the edge there are 80-90 pores in one transverse series 

 of the half-ambulacrum. 



The ambulacral vessels as they are distributed to the pores lie 

 sometimes free, sometimes covered in ambulacral chambers. In 

 Clypeaster scutiformis the series of pores are uncovered through 

 the whole ambulacral area, but the ventral and dorsal portions of 

 the shell are connected by numerous acicular pillars ; in Cly- 

 peaster Rangianus, placunariits, and Arachnoides placenta, the 

 series of pores lie for the most part free, but at the edge of the 

 disc they are enclosed in chambers, which, however, are wholly or 

 partially common to the abdominal and dorsal sides, inasmuch 

 as the walls of separation connect the abdominal and dorsal 

 parts of the shell. These parallel transverse chambers, of which 

 there are six in the Clypeasters in question, many (about twelve) 

 in Arachnoides, are traversed by a median longitudinal canal 

 which extends from the abdominal to the dorsal side. In this 

 lies the median ambulacral vessel, bending round from the 

 abdominal part of the shell to the dorsal portion, and giving off, 

 both in the ventral and in the dorsal part of its course, two 

 transverse branches to each chamber, so that every marginal 

 chamber has four vessels, two ventral and two dorsal, and as 

 many series of pores. Clypeaster' rosaceus, with a ventricose pe- 

 riphery, has no common dorso-ventral marginal chambers, but 

 the entire ambulacra lie between double walls, and the pennate 

 branches of the ambulacral vessels and their series of pores 

 are everywhere contained in ambulacral chambers or ducts. 

 Between the two transverse series of chambers there is also a 

 covered way for the trunk of the ambulacral vessel. In this 

 Dsedalian maze there are usually four chambers on every am- 

 bulacral plate, and from the mouth to the petaloid ambulacrum, 

 on each side about fifty chambers ; at their outer extremities 

 they open into the abdominal cavity. Every chamber contains 

 four series of pores, the number of pores in which is about 

 80-90 in the broadest part of the ambulacrum. The 2 x 50 



