230 



The b>chi iioplu tens a (Tore Is llir f^ieatesl diversity oi toiins of all iMliino- 

 (liTiii larva', and several well marked types are to be disliiimiished. II does 

 not imiuedialely appear wliieli of lliese represents the more primitive type. 

 Consideriiit*, howcvei'. the lad IIkiI Ihc hnvir in which the ixxly skeleton 

 in the liisl sta^e tonus a baskcl-sliiicliire, and which have in their second 

 stage a posterior transverse rod and more or less develo|)ed posterolateral 

 |)rocesses. are characteristic — so far as we know of the (lidarids, Dia- 

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

 l-A-hinoids. it can hardly be disputed that we have got to regard this larval 

 type as the more primitive form. Co nseq n c n 1 1 \ t he larval ty pe cha i- 

 acleristic of the family I-^chinidae s. str., with the elongated, club- 

 shaped l)()(l\ rods, with the recurrent rod rudimentary oi- absent, and with- 

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

 specialized and exceptional larval ty])e. it is therefore not at all 

 justifiable to make this larval type represent the Ecliinoid larva* in general, 

 as is done in most text-books. 



Characteristic of the larval body of the primaiy type are the vibralile 

 lobes; in the more specialized types, the larviv of the lu'hinida' s. str., and 

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

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

 the lobes are the epaulets occurring in the higher ty])es of the Hegularia. 



Both the vibratile lobes and the epaulets evidenll\ serve to increase the 

 floating power of the larva. 'Iliis object is atlaiiuMl to a still higher degree 

 in several lar\al foims of Regular h'.chini and Clypeastroids in which 

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

 these arms become actively movable. These larva', wiun lloating, keep the 

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

 disturbt'd. This is not yet an active swimming movement, the muscular 

 apparatus being loo sim|)le for performing regularly repeated movements. 

 Only one I'.chinoid larva appears to be able to swim actively, viz. the 

 remarkable Kchiiioplulciis Iraiisrcrsiis. in which a complicate muscular 

 system is (levelo])ed, as described above (comp. lig. 'M. p. <S(S), the body- 

 skeleton being most extraordinarily adapted for serving as a support to 

 the muscles. 



The four main arms, the postoral and postt'iodoi'sal, arc liu' most di\i'rs- 

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

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

 lu-hinoplulcus liansi>eisus, 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 Mespilia-\i\r\i\. Ihc anterolateral and preoral arms 

 are very uniform in ciiaracler throughout the whole class. The |)Ostero- 



