146 



INTRODUCTION TO ZOOLOGY 



CHAP. 



The sexes are distinct ; the sexual organ, ovary 

 Reproduc- or S p erm ary, ii es j n the visceral mass at the upper 

 part of the foot, but only by making a micro- 

 scopic examination can we tell, with certainty, which of the 

 two is present in any one individual. The eggs are discharged 

 from the ovary and are passed into the outer gills, which 

 become in consequence very greatly distended, and are known 

 as the "brood-pouches." The eggs 

 in this position are fertilised by the 

 sperm cells carried in by the in- 

 halent water current. This takes 

 place in the summer, and the 

 development of the eggs goes 

 steadily on within the brood-pouch 

 until the following spring, the 

 embryos being nourished all this 

 time by a mucous substance se- 

 creted by the gills. Even by the 

 autumn of its first year of life, the 

 little embryo has reached what is 

 known as the Glochidium stage, in 

 which it has a bivalve shell with a 

 sharp-toothed hook at the apex of 

 each triangular valve (Fig. 92). 

 Each valve is perforated by many 

 minute pores. The shell is lined 

 by a bi-lobed mantle which bears 

 little sensory cells, and between the 

 two valves of the shell runs a strong 



B. 



seen from below with shells 

 gaping widely. 



FIG. 92. 



A, Glochidiura of Anodon seen 

 from the side ; I), byssal thread ; 

 t, tooth at margin of shell; s, 



sensory papillae. B, Glochidium muscle, by means or which the shell 

 can be closed. From a little papilla 

 on this muscle projects a long, 

 coiled, sticky thread, the byssus. In February or March, 

 these Glochidium larvae are discharged from the brood-pouch, 

 pass out into the water usually in the exhalent water 

 current and are carried along by any current in the water 

 outside ; or they may sink gradually, often catching, by means 

 of the byssal threads, on to some weed and then remain- 

 ing supported in mid-water. Should a fish come near them, 

 they become very much agitated, and clap the valves of their 

 shells violently ; this causes the byssus to extend more, and it 



