X'A 



lateral arms, always the most prominent of [hv Ophioplutcus-arms, are 

 developed as hue arms only in Arbaeiidsand Spalanqoids. in Ihe rest of the 

 larvte they have the slia|)e of broad. earsha|)ed lobes or are entirely lacking. 



As regards (he skeleton it is a noteworthy fact that the fenestrated 

 rods rrpii'scnl a primary strncture, as nnisl be concluded from Hie 

 fact that tiiis type of rods (always confined to the four main arms) is found 

 in the larva- of tiu- more primitive forms, simple postoral and poslerodorsal 

 rods, upon Hie whole, occurring only quite exceptionally (Lijlerhimis. 

 Krhinubrissus) outside the family lu'hinidie, the lai-viie of which are al.so 

 otherwise among the most specialized of all Kchiiu)i(i-larva'. In some cases 

 (e. g. Temnopleurid larva, species c. fig. 22. p. <>1) these rods begin as 

 fenestraltMJ, but leiininale as simple rods. In h'.vcchimis rhlomlinis they 

 appear to have only two component rods. 



The preoral and anterolateral rods are always simple, more or less 

 spinous, presenting no features of special interest. The dorsal arch al.so 

 affords little diversity: the long lateral processes from it. supporting lobes 

 of llic dorsal side of the body (e. g. Temnoj)leurid-larva, s])eciesc; Echiiw- 

 melnt liKiinlcr), aie very probably homologous with the anterodorsal rods 

 of Spatangoid-larvse. — The primitive type of the body skeleton is, as 

 stated above, that in which the body rod and recurrent rod unite so as 

 to form a basket structure. This has been given up only in the Temno- 

 pleurida^ and h2chinida\ where the body rod has been more or less specially 

 (l('\eloped. and, (■.\ce|)li()iially, in the genus Lylechimis of the Toxopneu- 

 slidae. A special development of the basket structuii' is found in the I'.ciiiuo- 

 metridae, where the recurrent rod has become double, and in the Cllypea- 

 stroids, where the posterior part of the "basket" often develops into a 

 large, fenestrated |)lale. 



The element of the Hchinopluteu.s-skeleton dis|)laying the greatest divers- 

 ity is the posterior transverse rod, oi- rather the branches from its i-nds. 

 the posterolateral rods. These may be simple rods, now very short, now 

 extraordinarily long (1*1. \'l, Fig. 1), or branching structures of ex(|uisite 

 form (e. g. Temnopleurid-laiva. species a. fig. 20. p. .)8; Heliocidaris luhci- 

 cuUtla, fig. 21. p. (i.i). While it has completely disappeared in the Clypea- 

 stroid-larvte. it has been fuilher specialized in the Spatangoid-larvae, car- 

 rying the uii|)aiix'd posterior process, so characteristic of that larval t>pe. 



The ventral transverse rods are generally of a very uniform character in 

 the Regularia, whereas in the Clypeastroids and Spatangoids they are 

 somewhat specialized, being reduplicated in the former, often assuming a 

 broad, flat shape in the latter. This apparently unimportant skeletal ele- 

 ment has been nuide the starting point of an extraordinary development 

 in Eihinupluleus Iransnersiis. where it is transformed into a complicate 



