while others, such as l&chnodtiton 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, Boreocliiton cinereus, etc.). In 

 most specjes 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. As a 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- 

 pleurus, etc.) to nine (Cryptoplax larraeformis : 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 

 appendages ; or, as in Ischnochiton 

 magdalenensis, they may be laid 

 nearly 

 These 



in strings containing 

 200,000 eggs (Fig. 34). 

 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 polii) : 

 in Hemiarthrwm setulosum the 

 embryos are protected in this 

 manner until the eighth shell- 

 valve is formed. In Callistochiton IschMon magda , enensiSt female> laying 



viviparUS the Ova are developed eggs, hind part, ventral aspect. /, foot; g, 

 \ . i ,. ,i ,1 j ovary; g.d, glandular oviduct; 0.17, genital 



111 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. 



en?- 



PIG. 34. 



