Prof. J. Muller on the Structure of the Echinoderms . 9 



ambulacra! area?, without suckers or any relation to the ambu- 

 lacral grooves of the calyx, the suckers being situated in the 

 ambulacral grooves of the calyx and of the arms. 



I shall return to the antambulacral pores of the calyx of 

 Caryocrinus, Hemicosmites, and Echinospharites, in a particular 

 section devoted to the ambulacra of the Crinoids in general, and 

 especially the group of Cystidea of Von Buch. 



In an Echinoderm which remains antambulacral close up to 

 the mouth, and develo])es arms only from the oral part of the 

 calyx, we have at its maximum that condition which in the 

 Echinidce is at its minimum. To borrow the phraseology of the 

 ' Natur-philosophie,^ we may say that the calyx of a Pseudo- 

 crinitesj Agelocrinites, Echinospharites, Echinoencrinus, is the 

 apex of an Echinus ; it is, however, an expansion of the apex 

 large enough to enclose the whole intestines of the animal, while 

 in the Echinus these are invested for the most part by the ambu- 

 lacral zone of the perisoma. 



The region of the ambulacra may be reduced to a circlet of 

 suckers about the mouth, as in the Synaptce, CJiiridota, Mol- 

 padice, among the Holothuriadce. In the last instance, the antam- 

 bulacral area is only so large externally, Jager having demon- 

 strated that the ambulacral canals of the Synapta have as wide 

 a distribution internally as those of the other Holothuria. 



Although the ambulacra converge towards the oral pole, yet 

 they are not in all cases provided with suckers up to the middle 

 of the oral surface. In Echinus, for example, the ambulacra are 

 interrupted around the oral area ; and in many Crinoids, also, 

 the oral area between the ambulacral sides of the arms, instead 

 of presenting a continuation of the ambulacral grooves to the 

 mouth, is covered uniformly with plates, as in Actinocrinus. 



To the ambulacral system of canals, taken in its widest sense, 

 belong, together with the suckers, the ambulacral canals of the 

 radii, with their branches to the suckers and their ampullae, — the 

 circular canal which unites the five ambulacral canals around the 

 mouth and a few appendages connected with it. The system of 

 ambulacral canals exhibits ciliary motion over the whole extent 

 of its internal walls and is everywhere closed, if we except the 

 porous commencement of the sand-canal in the madreporic plate. 

 This canal, extending from the madreporic plate to the circular 

 canal, has now been observed in all forms of Echinoderms, with 

 the exception of the Crinoids. Its porous commencement either 

 opens externally as a madreporic plate in the Asteridce, Euryala, 

 EchinidcB; or is turned towards the abdominal cavity, as the 

 porous calcareous sac of the Holothuriada, which possesses an 

 external aperture only in the young state. In the Ophiurida^, 

 also, the calcareous sac is hidden in the cavity of the body. 



