Echinoidea. The Echinus larva ([fig. 263]) has a Pluteus form like that of the Ophiuroids, and in most points, such as the presence of the anal lobe, the ciliated band, the provisional skeleton, etc., develops in the same manner. The chief difference between the two Pluteus forms concerns the development of the lateral arms. These, which form the most prominent arms in the Ophiuroid Pluteus, are entirely absent in the Echinoid 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 larvæ. They are originally 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).
Fig. 265. Lateral and ventral view of a larva of Strongylocentrus.
(From Agassiz.) General references as in fig. 264.
b. dorsal opening of madreporic canal; e´. posterior arms; e´´´. anterior arms; eIV. anterointernal arms.
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 (g´). A fourth pair (not represented in Ophiuroids) appears on the inner side of the anterior pair forming an anterointernal pair (e), and in the Spatangoid 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 processes, 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[224]. 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 larvæ of Echinoids have been described by Agassiz[225].
The development of the permanent Echinus has been chiefly worked out by Agassiz and Metschnikoff.