ECHINODERMA TA—ONTOGEN Y 



509 



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

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

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

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

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

 l^ostanal process projecting backwards from the posterior end of tlie anal area ; its 

 tip may carry a ea[) 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 Ecliinoidea 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) frinu 

 the venti-al side. 1, Ciliated "epaulettes " ; nnt, an- 

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



jiont 



Fa;. 401.— Spatangid larva (Pluteus) 

 from the ventral side. ] , 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 PhUeus of Echinus has eight arms or processes, and at the bases of each ot 

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



The larvii; of Arbacia and Spatangus [Vig. 401) have no ciliated "epaulettes," 

 but Arbacia has two and S2Mtangus 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 Antrrhm 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 Hagella. The larva, in swimming, has the 

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

 forwards. 



