xvi ECHINODEKMATA 533 



remains simple ; and, secondly, the loop-like outgrowths of this band 

 remain much shorter than in the Bipinnaria larva. They are usually 

 called "processes," but as they are homologous with the arms of other 

 types of Echiiioderm larvae we shall call them by the same name. 



We find prae-oral arms (pr.o.a, Fig. 398) in front of the mouth 

 on the side of the prae-oral loop ; at the sides of the anal loop post- 

 oral arms (p.o.a, Fig. 398); and each side of the longitudinal band 

 carries an antero-dorsal, an intermediate-dorsal, and a postero- 

 dorsal arm (a.d.a, int.d.a, pd.a, Fig. 398). Where the anal loop 

 originates from the longitudinal band there is a well-marked postero- 

 lateral arm on each side (pl.a, Fig. 398). This arm is the one 

 which, from a fancied resemblance to the human ear, suggested the 

 name Auricularia to Johannes Miiller (1850), by which name the 

 larva is known. 



AURICULARIA LARVA 



From the foregoing description it is obvious how closely the 

 Bipinnaria and Auricularia larvae resemble one another in external 

 appearance, but the Auricularia larva differs widely from the Bipin- 

 naria in its metamorphosis, and indeed in the internal changes 

 which precede metamorphosis. 



The coelomic vesicle is at first situated in the mid-dorsal line ; it 

 shifts to the left, and then divides into anterior and posterior portions. 

 The posterior vesicle divides into right and left posterior coeloms, 

 which apply themselves to the sides of the larval stomach. This 

 change has been observed only by Metschnikoff (1869). 



The anterior vesicle does not, however, as Selenka and Metschnikoff 

 imagined, become directly converted into the hydrocoele ; Bury has 

 shown that it becomes divided into dorsal and ventral parts by a 

 constriction (1896). The dorsal part, which communicates with the 

 exterior by the primary madreporic pore, is the left anterior 

 coelom; the ventral portion is the rudiment of the hydrocoele, 

 and the connecting narrow part is the stone canal. Neither right 

 anterior coelom nor right hydrocoele are developed. 



In the Auricularia larva, then, in spite of its outward bilateral 

 symmetry an inner asymmetry is early evident. The hydrocoele has 

 the form of a hoop whose plane is the frontal plane of the larva, and 

 it lies at the left side of the oesophagus. From its outer surface there 

 arise five lobes, which are not the rudiments of the radial canals of 

 the water-vascular system, but of the primary buccal tentacles 

 (l v 2 V 3 V 4. V 5 P Fig. 398). Alternating with these lobes there appear 

 five much smaller lobes (1, 2, 3, 4, 5, Fig. 398), which are the rudiments 

 of the radial water-vascular canals. 



On these facts Semon's whole hypothesis of the phylogenetic 

 relationships of the classes of Echinodermata is built. He regards 

 the first five lobes as equivalent to the five primary lobes of the 

 hydrocoele in the Bipinnaria, Ophiopluteus, and Echinopluteus larvae, 

 and therefore equivalent to the radial canals in the Asteroidea, 



