Prof. J. Miiller on the Structure of the Echinodenns. 23 



In the genera Mellita, Lobophora and Encope, the lateral twigs 

 of the ambulacral vessel become immediately hidden in a super- 

 ficial labyrinth of fine canals in the shell, which form ambulacral 

 galleries, and are here and there connected with deeper sinuses 

 open towards the abdominal cavity. 



In these genera the pore -fascise, at a short distance from the 

 mouth, divaricate in consequence of their dichotomous divisions 

 from the middle of the ambulacrum, in which the trunk of the 

 ambulacral vessel lies. No branches of the ambulacral vessel 

 follow the coui'se of the two pi'incipal branches of the pore-fasciae, 

 but the pores of these two large pore-fascise are supplied by vas- 

 cular twigs, which are directed transversely to them, and partly 

 arise directly from the median ambulacral trunk, partly have a 

 pennate origin from branches of the latter. But all these twigs 

 of the ambulacral vessels run in narrow ambulacral galleries, 

 which occasionally anastomose and give off transverse offsets in 

 close succession which meet the lateral ambulacral fasciae, and 

 then penetrate in part transversely, in part obliquely, to their 

 pores. From this source then the pores receive their twigs. In 

 Mellita quinquepora I could follow out the branches of the am- 

 bulacral vessels from the trunk into the galleries, to theii very ulti- 

 mate ramuscules. In this manner also the many secondaiy 

 branches of the pore-fasciae are supplied. But, I repeat, the ra- 

 mifications of the pore-fasciae and those of the branches of the 

 ambulacral vessels are totally diflerent. 



The ambulacral galleries of Mellita, Lobophora and Encope 

 occupy a thin superficial layer of the shell and are distinguished, 

 by the nan-owness of the canals, from the other deeper sinuses of 

 the shell common in this genus. 



I am unable to confirm the statement that caecal processes of 

 the intestine lie in these sinuses in Mellita quinquepora and other 

 Clypeasteriike. In the former species, as well as in Mellita heoca- 

 pora, in Lobophora, Clypeaster, and, in fact, in all the genera 

 which I have examined, the intestine has no caeca, and passes by 

 the sinuses and apertures of the ambulacral chambers without 

 giving off any processes whatever, accompanied at its outer edge 

 by a great vessel, as in the regular Sea-urchins, and fastened by 

 a mesentery. In Mellita and Lobophora a portion of the lobules 

 of the sexual organs passes into the sinuses of the shell. 



The innumerable muscular organs, subject to voluntary con- 

 trol, on the back of the shell of a Sea-urchin, such as the 

 suckers, pedicellariae, and the muscles of the spines, receive 

 their nerves from the ambulacral nervous trunk lying in the in- 

 terior of the shell, whose branches accompany the branches of 

 the ambulacral vessel. The nervous trunks of the five ambulacra, 



