288 Prof. S. Loveii on the Structure of the Echinoidea. 



pore, loses itself on the outside of tlie calcareous layer, beneath 

 the overlying connective tissue, in a great number of branches, 

 which run through the anterior part of the plate in a radiating 

 and diagonal direction in order to distribute themselves to the 

 interradial radioli and other external parts attached to the 

 plate. This branching is most distinctly seen on the third 

 plate of the bivium situated near the sternum, which is freer 

 from spines than the second. 



The s^jheeridia make their appearance, seemingly, later than 

 the spines and pcdicellaria3 in very young Spatangidas {Bris- 

 sopsis hjrifera^ Echinocardium ovatum)^ first one alone in tlie 

 single pcristomial plate of the ambulacra, then one on the second 

 plate, and so forth ; all in accordance with the order which 

 prevails in the disposition of the ambulacral plates throughout 

 the whole class, and which, at least in all recent Echinoidea, 

 may be expressed by a formula common to all. This order is 

 as follows : — 



If Ave hold a Spatangus of any species with the mouth 

 turned upwards and the unpaired interradium backwards, and 

 count the ten peristomial plates of the ambulacra going from 

 left to right (that is to say, from the animal's right to its left 

 side) round the buccal aperture of the test, marking, in each of 

 the ambulacra I., II., III., IV., V., the plate we first come to 

 with the letter a, and the second with h, Ave shall find that the 

 plates I. a, II. a, III. b, IV. a, V. b are larger and bear tAvo pores 

 and tAvo tentacles, whilst I. b, II. 5, III. a, IV. b, and V. a are 

 smaller and furnished Avitli only one pore and one tentacle. 

 Assuming that here each ambulacral plate has originally only 

 one tentacle and one jDore, the former series of plates should 

 consequently, although no suture can be detected, be composite 

 and binary, and the latter simple and primary, like all the other 

 ambulacral plates. The Cassidididce behave in exactly the 

 same manner ; in the binary plates one of the pores is situated 

 in the outer horn of the plate. That in the Chjpeastridce the 

 peristomial plates of the ambulacra folloAV the same rule is seen 

 from their unequal size — I. «, II. «, III. b, IV. «, and V. b being 

 larger than I. &, II. ??, III. «, IV. b^ and V. a ; and Clypeaster 

 rosaceus has in the smaller plates one, and in the larger ones 

 two tentacular pores, Avhich differ by their size from the 

 numerous pores for locomotor tentacles. If an Ecliinoneus 

 be held in the above-mentioned position and counted in the 

 same manner, the same arrangement is manifest. The peri- 

 stomial plates I. «, II. a. III. J, IV. a, and V. b are larger and 

 have tAvo pores, a complete double pore and one which is 

 marginal and half interruj)ted ; I. Z>, II. Z>, III. a, IV. b, and 

 V. a, on the contrary, are smaller and bear only one double pore. 



