10 Prof. J. Miiller un the Structure of the Echinoderms. 



These peculiarities have ah'eady been explained in the 'Anato- 

 mische Studien/ Besides the one or many sand-canals, there 

 are two other descriptions of organs united with the circular 

 canal of the Echinoderms, — the ' Polian vesicles ' and the ' race- 

 mose appendages/ Both these organs occur contemporaneously 

 in the Asterida (as is well known from Tiedemann^s investiga- 

 tions) and reappear in the Holothuriada. In the latter the 

 racemose appendages are recognizable as little vesicles, which 

 are connected in pairs, by short stalks, with the circular canal 

 of young Holothuricp, and contain very peculiar double granules 

 in constant tremulous motion, just like that of the otolithes of 

 the Gasteropoda. In the adult Holothurice these organs are 

 recognizable in the multitudinous small vesicles which beset the 

 circular canal and open into its interior. The Ophiuridce possess 

 four simple vesicles and no racemes. In the Spatangidte the 

 appendages are absent upon the circular canal. In the regular 

 Sea-urchins five stalked vesicles are connected with this canal ; 

 they have the same position as the Polian vesicles of the Ophiu- 

 ridcE, but their walls are cellular, and therefore allied to the 

 racemose appendages. In the Cbjpeasteridce only, however, is 

 the circular canal provided with many such appendages. 



Analysis of the Ambulacra of the Sea-urchins. 



After having expounded the anatomy of the central portion of 

 the ambulacral canals in the 'Anatomische Studien iiber die 

 Echinodermen ' (Archiv 1850), I set before myself the task of 

 analysing the ambulacra themselves. 



The double pores of the ambulacra lie, in the Sea-urchins, 

 either in the plates themselves or in their sutures ; the latter 

 occurring in the petaloid ambulacra of the Clypeasteridce. In 

 this case, either the ambulacral plates are similar, as in the peta- 

 loid ambulacra of Scutella, Laganum, Echinarachnius, Lobophora, 

 Mellita, Encope, Echinocyamus ; or alternately dissimilar, as in 

 Clypeaster and Arachnoides ; the smaller plates pass, in these, 

 only from the external to the internal pore, the larger extend from 

 the external pore to the internal suture. Upon this point Des- 

 moulins has already made some good observations. At the lower 

 extremity of the petaloid ambulacra, for the rest, the double 

 pores readily pass from the sutures on to the plates themselves. 



Desmoulins has thrown out the supposition, that originally 

 there is a special ambulacral plate for every double pore of the 

 Sea-urchins. In the Clypeasteridce, a number of pores corre- 

 sponding with locomotive feet occur on a single plate, and they 

 multiply in proportion to the growth of the plate ; but in all 

 other Sea-urchins, this hypothesis would appear to hold good. 

 The composition of the ambulacral plates in Echinus has hitherto 



