ECHINODERMATA. 567 



the Pluteus types in the different groups is not sufficient to generalise with 

 great confidence, a few points seem to have been fairly determined 1 . The 

 Plutei of Strongylocentrus (figs. 266 and 267) and Echinus have eight arms 

 and four ciliated epaulettes. The only Cidaris-like form, the Pluteus of 

 which is known, is Arbacia : it presents certain peculiarities. The anal lobe 

 develops a pair of posterior (auricular) appendages, and the ciliated ring, 

 besides growing out into the normal eight appendages, has a pair of short 

 blunt anterior and posterior lobes. An extra pair of non-ciliated accessory 

 mouth arms appears also to be developed. Ciliated epaulettes are not 

 present. So far as is known the Clypeastroid larva is chiefly characterized 

 by the round form of the anal lobe. The calcareous rods are latticed. In the 

 Pluteus of Spatangoids there are (fig. 263) five pairs of arms around the 

 mouth pointing forwards, and three arms developed from the anal lobe 

 pointing backwards. One of these is unpaired, and starts from the apex of 

 the anal lobe. All the arms have calcareous rods which, in the case of the 

 posterior pair, the anterolateral pair, and the unpaired arm of the anal lobe, 

 are latticed. Ciliated epaulettes are not developed. 



Viviparous larvaa of Echinoids have been described by Agassiz 2 . 



The development of the permanent Echinus has been chiefly worked out 

 by Agassiz and Metschnikoff. 



In the Pluteus of Echinus lividus the first indication of the adult arises, 

 when three pairs of arms are already developed, as an invagination of the 

 skin on the left side, between the posterior and anterolateral arms, the 

 bottom of which is placed close to the water-vascular vesicle (fig. 266 B, it/}. 

 The base of this invagination becomes very thick, and forms the ventral disc 

 of the future Echinus. The parts connecting this disc with the external 

 skin become however thin, and, on the narrowing of the external aperture of 

 invagination and the growth of the thickened disc, constitute a covering for 

 the disc, called by Metschnikoff the am n ion. The water-vascular vesicle 

 adjoining this disc grows out into five processes, forming as many tube feet, 

 which cause the surface of the involuted disc to be produced into the same 

 number of processes. The external opening of the invagination of the disc 

 never closes, and after the development of the tube feet begins to widen 

 again, and the amnion to atrophy. Through the opening of the invagination 

 the tube feet now project. The dorsal and right surface of the Pluteus, 

 which extends so as to embrace the opening of the madreporic canal and 

 the anus, forms the abactinal or dorsal surface of the future Echinus 

 (fig. 267, a}. This disc fits on to the actinal invaginated surface which arises 

 on the left side of the Pluteus. On the right surface of the larva (dorsal of 

 permanent Echinus) two pedicellarias appear, and at a later period spines 

 are formed, which are at first arranged in a ring-like form round the edge of 

 the primitively flat test. While these changes are taking place, and the two 

 surfaces of the future Echinus are gradually moulding themselves so as to 



1 Vide especially Mailer, Agassiz, and Metschnikoff. 



2 For viviparous Echini vide Agassiz, Proc. Amer. Acad. 1876. 



