4:72 EC HI NO ID P LUTE US. 



absent in the Echmoid Pluteus, which accordingly has, as a rule, a 

 much narrower form than the Ophiuroid Pluteus. 



A pair of ciliated epaulettes on each side of and behind the ciliated 

 ring is very characteristic of some Echinoid larvae. They are origi- 

 nally developed from the ciliated ring (fig. 266 A and B, v"). The 

 presence of three processes from the anal lobe supported by calcareous 

 rods is characteristic of the Spatangoid Pluteus (fig. 263 E). 



The first two pairs of arms to develop, employing the same names as in 

 Ophiuroids, are the anterior attached to the oral process (fig. 263 C, d) and 

 the posterior pair (e). A pair of anterolateral arms next becomes developed 

 (</'). A fourth pair (not represented in Ophiuroids) appears on the inner 

 side of the anterior pair forming an anterointernal pair (e), and in the Spa- 

 tangoid Pluteus a fifth pair may be added on the external side of the 

 anterior pair forming an anteroexternal pair (g). 



Each of the first-formed paired calcareous rods is composed of three pro- 

 cesses, two of which extend into the anterior and posterior arms; and the 

 third and strongest passes into the anal lobe, and there meets its fellow 

 (fig. 265). A transverse bar in front of the arms joins the rods of the two 

 sides meeting them at the point where the three processes diverge. The 

 process in the anterolateral arm (fig. 266 B) is at first independent of this 

 system of rods, but eventually unites with it. Although our knowledge of 

 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 charactei'ized 

 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 point- 

 ing backwards. Oue 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 pos- 

 terior pair, the auterolateral pair, and the unpaired arm of the anal lobe, are 

 latticed. Ciliated epaulettes are not developed. 



Viviparous larvae 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, 

 w'). 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 



1 Vide especially Miiller, Agassiz, and Metschnikoff. 



8 For vivipaious Echini vide Agassiz, Proc. Amcr. Acad. 1876. 



