24 Echinoderma. 



separate rings. The Echinothuridae agree with the latter group in the plates on the 

 buccal membrane being modified coronal plates, while interambulacral plates are 

 absent as in young Cidaridae. The ensheathing of the spines of the Echinothuri- 

 dae is an embryonic character of the Euechinoidea. The ontogeny of Cidaris 

 shows that the perisome is more moveable in young individuals, while their corona 

 consists of numerous small and isolated plates which subsequently become united, 

 i. e. they pass through an Echinothurid stage. So also palaeontologically, Echi- 

 nus, Diadema, and Asthenosoma form an unbroken series leading back to the Pa- 

 laeechinidae, the number and the irregular character of the plates gradually in- 

 creasing in the older forms. 



Duncan gives a minute description of the structure of the ambulacral plates 

 and branchiae in various genera of the Temnopleuridae. He concludes that 

 the so called geminous or reverse condition of the ambulacral pores is not of the 

 least physiological importance. Four sets of muscular fibres are visible in the 

 tentacles of Temnopleurus and Salmacis: 1) the circular fibres of the cup; 

 2) the concentric and radiating fibres at the top of the tentacular cavity and base 

 of the cup ; 3) the outer layer of circular fibres ; 4) the inner layer of longitudinal 

 fibres. The calcareous circlet of T. is in 4 parts, and that of S. in 6 or 7. In 

 both genera the abactinal tentacles are larger and more baggy than the actiual 

 ones. - - Four kinds ofpedicellariae occur in 7 1 ., large and small tridactyle. 

 globiferae, and triphylta, which last are absent on the buccal plates, though pre- 

 sent there in S. 



Loven gives the name epistroma to a growth of calcified protuberances which 

 is superposed on the proper surface of the plates of an Echinid and is especially 

 developed in the Arbaciae. It is an independent system antagonistic to that of 

 the tubercles, and does not serve as support to organs of any kind. Near the 

 tubercles it consists of crowded protuberances which are either sessile or sub- 

 pedunculate, while in the parts of the test devoid of tubercles it appears as a con- 

 tinuous finely striated layer with radiating rows of minute glossy knobs imbedded 

 in it. It is possibly a modification of the membranous envelope of the Pluteus. 

 which calcifies independently of the underlying skeletal plates. In the calyx 

 of the young Arbacia it forms a heterotropous pentagon like that which was for- 

 merly constant for life in the Saleniae ; and it seems to have descended to them 

 from the early Crinoids such as Callicrinus, which has protuberances on its calyx 

 recalling those of many Asterids. 



John discusses the boring habits of some sea-Urchins and concludes that they 

 are due to the need of protection from the waves on exposed and rocky coasts. 

 The boring is principally effected by the jaws with the help of the spines during 

 rotary movements of the body. Crystals and other large fragments in the rock 

 are loosened round their edges, and the digestive' tube of the Urchins is filled 

 witli rock-debris. 



VI. Holothurioidea. 



See also the Sarasins ( 3 ), supra, p 4 and p 22. 



Semon has studied the ontogeny of Synapta digitata after the commencement 

 of the Auricularia-stage, and finds in addition to the longitudinal ciliated band 

 an adoral band, which closely surrounds the mouth beneath the pre-oral lobe and 

 sends a loop downwards into the fore-gut. The ciliated bands and the lateral 

 nerve bands of Metschnikoff [see Bericht for 1884 I p 171] are situated in super- 

 ficial grooves bounded inferiorly by flattened mesenchym cells, which also extend 

 as a cutis over the whole surface of the larva beneath a delicate epidermis. The 



