THE AMPHINEURA 51 
while others, such as Ischnochiton magdalenensis, are nocturnal in 
habit. ; 
5. Reproductive Organs.—All the Polyplacophora are unisexual. 
The gonads are paired and symmetrical in Nuttalochiton hyadesi, but 
in all other species the gonad is single and median. It lies on the 
dorsal side of the body, between the aorta and intestine, and 
extends from the anterior end to the pericardium (Fig. 25, II). 
It is transversely wrinkled, and the ovary is frequently of a greenish, 
the testis of a red colour (Chiton polii, Boreochiton cinereus, ete.). In 
most species the ova are enclosed in a cellular follicle. 
The paired genital ducts arise from near the posterior end of 
the gonad on the dorsal surface. Each duct is twice bent on itself, 
being directed first forwards, then backwards, and finally transversely 
toward the external opening. Asa rule the gonaduct runs dorsad 
of the pallial nerve-cord, and its external opening is external to the 
cord, but in Boreochiton ruber, B. marmoreus, and Katharina it runs 
under the cord and opens internally to it. In the female the 
oviduct has a thick glandular investment (Fig. 25, III). The genital 
aperture is situated in front of the renal aperture, between two of 
the posterior gills. There may be from one (Boreochiton, Lepido- 
plewrus, ete.) to nine (Cryptoplax larvaeformis: Fig. 26) gills between 
the genital and the renal apertures. 
III. EMBRYOLOGY. 
The eggs may be laid separately, in which case they are invested 
by a chitinous envelope fre- 
quently provided with spinous f be mh 
appendages ; or, as in [schnochiton : 
magdalenensis, they may be laid : 
in strings containing — nearly 
200,000 eggs (Fig. 34). These 
strings are not attached, and 
readily break into fragments. 
In many cases the ova are re- 
tained in the branchial furrow 
of the mother and undergo their 
development there (Chiton polit) : 
in Hemiarthrum  setulosum the 
embryos are protected in this 
manner until the eighth shell- 
valve is formed. In Callistochiton Ischnochiton magdalenensis, female, laying 
‘viviparus the ova are developed eggs, lind part, ventral aspect. f, foot; 9, 
: a ovary; g.d, glandular oviduct; 0.g, genital 
in the oviduct of the mother, and orifice ; ov, ova; pa, mantle. (After Heath.) 
there is no larval stage. 
The segmentation is total and, in the early stages, regular. 
Fic. 34, 
