230 



The Echinopluteusaffordsthe greatest diversity of forms of all Echino- 

 derm larvse, and several well marked types are to be distinguished. It does 

 not immediately appear which of these represents the more primitive type. 

 Considering, however, the fact that the larvae in which the body skeleton 

 in the first stage forms a basket-structure, and which have in their second 

 stage a posterior transverse rod and more or less developed posterolateral 

 processes, are characteristic — so far as we know — of the Cidarids, Dia- 

 dematids and Arbaciids, that is to say of the more primitive forms of 

 Echinoids, it can hardly be disputed that we have got to regard this larval 

 type as the more primitive form. Consequently the larval type char- 

 acteristic of the family Echinidae s. str., with the elongated, club- 

 shaped body rods, with the recurrent rod rudimentary or absent, and with- 

 out a posterior transverse rod or posterolateral processes, is a highly 

 specialized and exceptional larval type. It is therefore not at all 

 justifiable to make this larval type represent the Echinoid larvae in general, 

 as is done in most text-books. 



Characteristic of the larval body of the primary type are the vibratiie 

 lobes ; in the more specialized types, the larvae of the Echinidae s. str., and 

 of the Spatangoids these lobes have disappeared, while the Clypeastroid- 

 larvae have retained them to some degree. A further specialization from 

 the lobes are the epaulets occurring in the higher types of the Regularia. 



Both the vibratiie lobes and the epaulets evidently serve to increase the 

 floating power of the larva. This object is attained to a still higher degree 

 in several larval forms of Regular Echini and Clypeastroids in which 

 muscles connect the lower ends of the rods of the four main arms, so that 

 these arms becortie actively movable. These larvae, when floating, keep the 

 four main arms in a more or less horizontal position, raising them when 

 disturbed. This is not yet an active swimming movement, the muscular 

 apparatus being too simple for performing regularly repeated movements. 

 Only one Echinoid larva appears to be able to swim actively, viz. the 

 remarkable Echinopluteus transversus, in which a complicate muscular 

 system is developed, as described above (comp. fig. 34, p. 88), the body- 

 skeleton being most extraordinarily adapted for serving, as a support to 

 the muscles. 



The four main arms, the postoral and posterodorsal, are the most divers- 

 ified of the larval arms. They are always rather long, but sometimes (Dia- 

 dematidae) attain a very great length. This is carried to an extreme in 

 Echinopluteus transversus, as regards the postoral arms, while the postero- 

 dorsal arms have disappeared. In some forms these arms are broad and 

 flat, especially so in the Mespj7/a-larva. The anterolateral and preoral arms 

 are very uniform in character throughout the whole class. The postero- 



