VIII 



ECHINODERMA TAONTOGEN Y 



509 



cesses are of two kinds. One kind, which belong to the region of the circumoral 

 ciliated ring, are paired, and diverge in a forward direction. Two arms an; 

 distinguished by their constant occurrence and special length ; these belong to the 

 posterior and lateral region of the circumoral ciliated band. Opposed to these paired 

 processes of the circumoral ciliated band pointing anteriorly is an unpaired, posterior, 

 postanal process projecting backwards from the posterior end of the anal area ; its 

 tip may carry a cap of cilia. 



In Ophiuroidea in which care of the brood occurs, the typical larval forms are not 

 developed. 



4. Echinoidea (Figs. 400 and 401). The larva of Echinoidea agrees to a great 

 extent with that of the Ophiuroidea, and is, like it, known as the Pluteus. The only 



FIG. 400. Larva of an Echinid (Pluteus) from 

 the ventral side. 1, Ciliated "epaulettes " ; ant, an- 

 terior ; post, posterior ; dcx, right ; sin, left. 



post 



FIG. 401. Spatangid larva (Pluteus) 

 from the ventral side. 1 , The three processes 

 of the anal area. 



important difference is that the two lateral arms which, in the Ophiurids, are the 

 most constant and the longest, seem to be altogether wanting in the Echinoidea. 



The Pluteus of Echinus has eight arms or processes, and at the bases of each ot 

 the four. posterior arms a ciliated " epaulette" (Fig. 400). 



The larvae of Arbacia and Spatangus (Fig. 401) have no ciliated "epaulettes," 

 but Arbacia has two and Spatangus three long posterior processes of the anal area, 

 which, like all the other processes, are supported by calcareous rods. 



Echinoids in which care of the brood occurs develop direct without meta- 

 morphosis. 



5. Crinoidea (Fig. 402). The free-swimming larva of Antedon is long and egg- 

 shaped. At the frontal pole, the thickened but somewhat depressed ectoderm (the 

 neural pit or plate) carries a tuft of long flagella. The larva, in swimming, has the 

 frontal pole, which corresponds with the anterior end of other Echinoderms, directed 

 forwards. 



