14 



AMPHINEURA. 



The development of the shell commences even during larval life. 

 But, since some Chitons leave the egg-membrane only at a very late 

 stage, the free-swimming stage is in their case of comparatively short 

 duration. The commencement of metamorphosis is indicated by the 

 compression and final degeneration of the cells of the ciliated ring 

 which from the first were distinguished from the surrounding ecto- 

 derm by their size and structure (Figs. 5 and 9 w). It must be 

 further mentioned that in the older larvae, two eyes are said to be 



Fig. 9. — Median longitudinal section through a young Chiton, just emerging from 

 the larval stage (after Kowalevsky). a, proctodaeum ; c, cuticle of the shell, the 

 darkened parts indicating, according to Kowalevsky, those points at 'which the 

 formation of the shell-plates is taking place ; eg, cerebral ganglion ; fd, pedal gland ; 

 k, calcified portion of the shell ; m, mouth ; vul, enteron ; mes, mesoderm ; r, radular 

 sac ; sr, sub-radular organ ; st, spines ; w, ciliated ring. 



present, which curiously enough lie, not in the actual pre-oral section, 

 i.e., in front of the ciliated ring, but behind it (Fig. 3 C). These 

 eyes are still to be found in the young Chiton, but, whereas they 

 were at first superficial in position, they are now found beneath the 

 epidermis (Fig. 4 0) and are consequently still nearer than before 

 to the lateral nerve-trunk.* 



* [Pelseneer (App. Lit. on Lamellibranchia, No. IV.) regards these 

 eyes of the larval Chiton as homologous with the cephalic eyes which have 

 recently been discovered in the adult Mytilidae under the branchial filaments. 

 In the larva, they are situated outside the velar area as is the case also in 

 Chiton, whereas the eyes of the typical Gastropods arise within the velar area. — 

 Ed.] 



