231 



lateral arms, always the most prominent of the Ophiopluteus-arms, are 

 developed as true arms only in Arbaciids and Spatangoids, in the rest of the 

 larvae they have the shape of broad, earshaped lobes or are entirely lacking. 



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

 rods represent a primary structure, as must be concluded from the 

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

 in the larvae of the more primitive forms, simple postoral and posterodorsal 

 rods, upon the whole, occurring only quite exceptionally (Lytechinus, 

 Echinobrissus) outside the family Echinidse, the larvae of which are also 

 otherwise among the most specialized of all Echinoid-larvae. In some cases 

 (e. g. Temnopleurid larva, species c, fig. 22, p. 61) these rods begin as 

 fenestrated, but terminate as simple rods. In Evechinus chloroticus 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 also 

 affords little diversity ; the long lateral processes from it, supporting lobes 

 of the dorsal side of the body (e. g. Temnopleurid-larva, species c; Echino- 

 metra lucunter), are very probably homologous with the anterodprsal rods 

 of Spatangoid-larvae. — 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- 

 pleuridae and Echinidae, where the body rod has been more or less specially 

 developed, and, exceptionally, in the genus Lytechinus of the Toxopneu- 

 stidae. A special development of the basket structure is found in the Echino- 

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

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

 large, fenestrated plate. 



The element of the Echinopluteus-skeleton displaying the greatest divers- 

 ity is the posterior transverse rod, or rather the branches from its ends, 

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

 extraordinarily long (PI. VI, Fig. 1), or branching structures of exquisite 

 form (e. g. Temnopleurid-larva, species a, fig. 20, p. 58; Heliocidaris tuber- 

 culata, fig. 24, p. 65). While it has completely disappeared in the Clypea- 

 stroid-larvae, it has been further speciahzed in the Spatangoid-larvae, car- 

 rying the unpaired posterior process, so characteristic of that larval type. 



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 reduphcated in the former, often assuming a 

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

 ment has been made the starting point of an extraordinary development 

 in Echinopluteus transversus, where it is transformed into a comphcate 



