246 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



are produced in groups or bunches, fastened together by their 

 heads. They are of the usual elongated type common amongst 

 the Mollusca. The head of the mature active sperm is narrow 

 and sinuous, its tail very long and slender. In the younger 

 stages the head may bear a clear disc of protoplasm, or this 

 may be distributed in the form of droplets along the course 

 of the tail. 



The cavity of the gonad, together with the renal and 

 pericardial cavities, constitutes all that remains of the true 

 coelom of Ajrtysia. The eggs and sperms are thus discharged 

 into the coelom before making their way to the exterior. 



The little hermaphrodite duct is thin-walled and is lined, 

 by a non-ciliated epithelium. According to the age of the 

 animal and the condition at the time of death, the duct contains 

 either pure sperm or mixed eggs and sperms. On reaching 

 the level of the accessory genital mass it has already been 

 noticed that a constriction occurs, and that the duct loops 

 round the spermatocyst. At this point the epithelial lining 

 of the duct becomes ciliated and a fold appears internally. 

 (Fig. 20d, lp.) This fold later becomes the sperm fold, which 

 is continuous from this point to the penis. As the duct 

 plunges into the accessory genital mass it becomes extremely 

 slender, and is difficult to trace in a hardened specimen. 

 Very shortly after entering the mass the duct suddenly enlarges 

 to form a large thin-walled chamber, whose inner walls are 

 strongly ciliated. This is the fertilisation chamber {fert. ch.). 

 Part of the albumen gland and of the winding portion of the 

 mucous gland must be dissected away to expose it. By 

 careful dissection it is seen that the sperm fold of the loop 

 passes along the right side of the fertilisation chamber and so 

 travels forward to the large hermaphrodite duct. Its course 

 will be traced later. 



The fertilisation chamber receives the discharge of the 

 albumen gland, whose walls are very thick, with parallel septa 



