10 



AMPHINEURA. 



We have already mentioned that we thought ourselves justified in comparing 

 the rudiments of the cerebral ganglia with the apical plate of the Annelidan 

 Trochophore (p. 6). The connection of this rudiment with the lateral 

 trunks of the nervous system in the larva cannot as yet be clearly established. 

 Kowalevsky speaks of the two organs as discontinuous. The cell-mass at 

 the posterior end of the body described as the visceral ganglion (Fig. 5, hg) is 

 (as regards its ganglionic nature) of the same significance and origin. 



Actual ganglia, indeed, are not developed in the nervous system of Chiton, 

 where we find the ganglionic cells distributed along the entire length of 

 the longitudinal commissures, nevertheless it appears from Kowalevsky's 

 ontogenetic researches that these trunks are greatly swollen both anteriorly 

 and posteriorly. The transverse commissures no doubt form in the same way 

 as the longitudinal trunks. 



When the embryo (Chiton Polii and C. olivaceus) has attained a 

 stage somewhat like the one described above, it leaves the egg and 

 becomes a free-swimming larva. At this stage, a fresh differentia- 

 tion makes its appearance in the form of a segmentation affecting the 

 dorsal surface. Here, seven consecutive segments may be dis- 

 tinguished, separated from one another by shallow grooves (Figs. 5 

 and 9). These structures are indications of the future shell ; this 



— -st 



B. 



A. 



R 



C. I 



Figs. 6 and 7. — Two sections through the mantle-epithelium of Chiton Polii (after 

 Blumrich). .1, epithelial papilla without spine ; B-l), papillae in various stages of 

 spine-formation ; E, later stage of the same ; bz, formative cells of the spines ; c, 

 cuticular covering of the body ; ep, mantle-epithelium ; m, mantle-tissue ; st, spines. 



eventually consists of eight plates, but the eighth segment only 

 appears at a later stage. 



The ectodermal skeleton in Chiton falls under two categories : (1) 

 the eight calcareous dorsal plates and (2) isolated spines and plates 

 situated anteriorly, posteriorly and laterally to the former (Fig. 8, 

 st). These spines are of special interest, being a characteristic feature 

 not only of Chiton but also of all other Amphineura. 



