STANDEN : THE ZEBRA-MUSSEL. 



87 



Fig. I: 



Fig. 1 — Young lar\a oi Di-eissensin, with bivalve shell {s.) and velum (r'f/.), seen from the side, 

 a, anus ; /«, stomach ; inu, mouth ; st?i, abductor muscle (from Korschelt, '92). 



swimming at full speed may be seen to stop suddenly and impart a 



spinning motion to its velum. At the slightest disturbance the velum 



retracts; the valves of the shell, which are only slightly open, snap 



Fig. 2. Fig. 3. 



s. 



5-— ya. 



vet 



Fig. 2. — Larva of Dreissensia, looking down Fig. 3. — Older larva of Dreissetisia, with 



from above upon the velum, which is velum greatly expanded, seen from 



fully expanded; /, pigmentation of the side; ?«, retractor muscles of the 



the velum (7't'/.) ; ^/, pigment beneath velum (iieL); pi, pigment; s, the 



the oral opening ; s, shell, which is two valves of the shell (from Kor- 



for the most part concealed by the schelt, '92). 

 velum. 



together and the larva sinks to the bottom until the danger is past. 

 The larvse swim in large numbers on the surface of the water, and I 

 was able to capture many in the canal at Newsham, near Preston, by 

 using a very fine net, on a perfectly calm and very hot June day. They 

 are, however, so very small that it is easy to mistake them for 

 rotifers — especially if the zone of cilia happens to be in active motion 

 during examination. 



Dr. Korschelt, in the course of his remarks, says: — "The chiei 



