542 COMPARATIVE ANATOMY chap. 



skeletal plates are also shifted. The horse-shoes formed by the five orals and the 

 five basals close, and form two flat rings or circles. The circle of the orals shifts 

 backward and on to the roof of the vestibule in such a way tliat the five plates together 

 form a pyramid, the truncated tip of which forms the centre of the vestibular roof, at 

 the extreme posterior end of the larva. The orals have thus shifted away fi-om the 

 region of the left (oral) coelom. 



The circle of basals which lies in front of (apically to) that of the orals forms a 

 pyramid in the body wall of the calyx, around the aboral ccelom, the truncated end 

 of this pyramid lying at the commencement of the stalk, or at the anterior (apical) 

 end of the calyx. The orals and the basals together form a pentagonal double 

 pyramid, truncated at both ends. At the truncated end of the basal pyramid, round 

 the uppermost (most posterior) joints of the stalk, lie the four or five small infra- 

 basals. The number of joints in the stalk increases, and the anterior body of the 

 larva becomes, as the stalk, more and more distinctly demarcated from the posterior 

 body or calyx, which has now become five-rayed. 



The orals and basals alternate with the primary outgrowths of the hydroccel, 

 i.e. they are interradially placed. If we indicate that primary outgrowth of the 

 hydroccel which, when the larva is viewed from the oral side, comes next iir the 

 direction followed by the hands of the clock, to the hydropore (which lies ventrally 

 to the right) as No. I, and those which follow in this direction as II, III, IV, and 

 V, and if, again, we indicate that oral or basal which lies in the interradius between 

 radii I and V as -the first, and those which follow in this direction as orals (or basals) 

 2 to 5, it can be proved that the hydropore, in the older stages of the attached larva, 

 is enclosed by the basal part of the first oral plate. In these stages it is also seen 

 that the infrabasals fuse to form a single plate, the centrodorsal, at the centre of 

 which there is an aperture for the passage of the chambered organ. 



During the first developmental periods which follow the attachment of the larva, 

 the sacculi appear. Five of these first arise exactly radially at the bases of the 

 middle tentacle of each group of tentacles, on the outer side of the circular canal. 

 These sacculi can be ontogenetically derived from groups of mesenchyme cells. 



4. The Stalked Larva after the Vestibule has been Perforated. 



(From five days to the sixth week after hatching, Fig. 454.) 



The calyx becomes more and more distinct from the stalk. 



The roof of the vestibule becomes ever thinner at its centre, an aperture finally 

 forming. Radial incisions run from this central aperture towards the peripheral base 

 of the roof, so that this latter becomes divided into five interradial lobes or valves, 

 each of which contains an oral plate. This pyramid of valves can open and shut. 

 The vestibule has opened outward. 



The five tentacles meanwhile lengthen and receive their papillse. They usually 

 project outwards from between the five oral valves. 



The definitive nervous system (which is oral and superficial) rises quite inde- 

 pendently of the larval nervous system, which entirely disappears. The first ap- 

 pearance of the nerve ring was observed very late, long after the perforation of the 

 vestibule. The ectoderm of the oral disc, i. e. the peripheral portion of the original 

 vestibular floor (the central part having sunk in to form the oesophagus), becomes 

 thickened in a ring which is bordered by tentacles, and here becomes multilaminar. 

 The cells of the deeper layer yield the nerve tissue. 



Neither the rudiments of the deeper oral nervous system nor those of the apical 

 system have as yet been certainly observed. 



