THE STAEFISH SOLASTER ENDECA. 47 



It will be remembered that in the young larva the cilia are longer and lash more 

 vigorously on a small area at the anterior pole than over the general surface of the 

 body (p. 15). This condition is transient. More important is the fact that the 

 epidermal cells of the area in question are distinctly longer than those which cover 

 the rest of the preoral lobe (PI. I. fig. 10 ; PI. III. figs. 33, 34), and that they remain 

 so throughout the whole of larval life. It is the basal parts of the cells which are 

 elongated, and between them a few fine horizontal strands like nerve-fibrils may be 

 detected, though not in such numbers as under the sucker. The area in question may 

 be called the apical field or plate. During metamorphosis the walls of the anterior 

 end of the larva are incorporated in great part with the oral surface of the young 

 Starfish, the actual epiblast-cells retaining their individuality throughout. What we 

 have called the apical field of the larva is carried to a position near the centre of the 

 oral face in the region of the earlier-formed hydroccele-pouches. Here the tissue 

 of the field is at any rate brought very near to the site of formation of a considerable 

 segment of the oral nerve-ring, which by extension at the two ends completes the 

 ring. Certain it is that, like practically all the other structures, the nervous system 

 is diff'erentiated latest in the last-formed rays. 



A nervous system more or less well developed has been described for larvae 

 belonging to all the five main divisions of Echinoderms. 



Thus in Asterias vulgaris Packard, P'ield (7, p. llo) noted the presence, at the apex 

 of the frontal field, of an area carrying elongated ciliated cylindrical cells with what 

 seemed to be nerve-fibres at their bases, and this area he compared with an apical plate. 



Macbride (15, p. 353) found underneath the " larval organ" of Asterina a layer of 

 fibrillse which he considered to be nervous in character. 



The larvae of Antedon and Synapta possess well-marked nerve-structures which, 

 in the case of the latter, help to form the nervous system of the adult. Nerve-tissues 

 have also been described in the echinid pluteus (Mortensen, 19; Macbride, 16 a) and 

 in the larval AmpMura (Russo, quoted from Bronn's ' Klassen,' II. 3, iii. p. 868). They 

 are said to be recognisable in the latter as far back as the gastrula stage, and to give 

 rise to the nervous system of the adult. 



Solaster, however, furnishes an example in which a larval nervous system of 

 the apical type comes very near to being carried over during metamorphosis into 

 the central nervous system of the adult. 



Muscular Tissue of Larva. — The larval muscular tissue is developed from the lining 

 of the various coelomic cavities, particularly from that of the anterior coelom. The 

 latter gives rise to the fibres which are in connection with the larval arms and with 

 the sucker, and to the oblique band which was referred to on p. 16 as in all probability 

 exercising a gubernacular action in efi'ecting the flexion and torsion of the preoral lobe. 

 The band in question is illustrated in PI. III. fig. 34. 



This band of fibrillse takes origin from the lining of the left side of the anterior 



