PHYLUM ECHINODEEMATA 



423 



In the Asteroidea the larva is either a bipinnaria (Fig. 348, 

 4 to 6) or a brachiolaria. The former has a series of bilaterally 

 arranged processes or arms ; the latter has, in addition, three 

 processes not developed in the course of the ciliated band and 

 used for fixation. The larva of Asterina, the development of which 

 has been described and illustrated on pp. 380-385, is a greatly 

 modified bipinnaria with the pre-oral lobe large and eventually 

 serving as a stalk, and the band of cilia confined to the edge of the 

 larval organ and devoid of the bilateral processes of the normal 

 bipinnaria. In at least one form the bipinnaria, developed in a 

 brood-pouch, adheres to the parent by means of the pre-oral lobe 

 which takes the form of a short stalk. In general the bipinnaria is 

 free-swimming and has a large pre-oral 

 lobe, the part of the ciliated band borne 

 on which becomes separated off as a 

 pre-oral loop (aor) from the rest or post- 

 oral loop (por). In the course of both 

 of these loops are the variously- 

 arranged paired processes. In both the 

 Ophiuroidea and the Echinoidea (Fig. 

 348, 7 to 9) the larva has the form 

 which is known as the pluteus. The 

 pluteus has a number of slender arms 

 directed forwards and supported by a 

 skeleton of delicate calcareous . rods : 

 the pre-oral lobe is reduced and the 

 ciliated band is undivided. The larva 

 of the Holothuroidea, the auricularia 

 (2 and 3), has a number of short pro- FIG 349 ._ Free _ 9wimming Iarva 



Cesses developed in the COUrse of the Antedon, from the left side. 



ciliated bands ; subsequently, in the 

 pupa stage, the ciliated bands become 

 broken up into a series of ciliated 

 hoops encircling the body. Of the 

 Crinoidea the development of Antedon alone is known. 

 Blastula and gastrula stages occur as in the Starfish, but the 

 history of the archenteron and its diverticula is widely differ- 

 ent, though the outcome is the same viz., the differentiation 

 of a primitive enteric canal, an anterior coelome, from which a 

 hydroccele becomes separated off, and a pair of ccelomic sacs. 

 The larva (Fig. 349) becomes barrel-shaped, and the pre-oral lobe, 

 which is not very conspicuous, develops an ectodermal thickening 

 with a tuft of sensory cilia. The vibratile cilia on the surface are 

 arranged in five transverse bands (7-F). Between the second and 

 third of these is a wide shallow depression, the vestibule or 

 stomodaeum (1), which does not communicate with the mouth. 

 After remaining in the free condition for a short time, the larva' 



I-V, ciliated bands ; ba\ to ba 5 , 

 the five basals ; or\ to or 5 , orals ; 

 1, vestibule ; 2, intestinal vesicle ; 

 3, right enterocoele ; 4, calcareous 

 joints of the stalk ; 5, pedal plate. 

 (From Lang, after Seeliger.) 



