20 ART. 5. N. YATSU : 



terior to this there is another thicker layer of compact nuclei, 

 being the clusters" of the heads of spermatozoa. The outer 

 surface of the latter layer has a ciliated appearance owing to 

 the presence of the tails. Spindle bodies force their way into 

 the narrow spaces of the testes, probably bearing some important 

 physiological relations to sperm cells. The above two layers of 

 testis are noted by Vogt and Jung ('88) in their Lehrbuch 

 (p. 724). 



The spermatozoon has a head, slightly pointed at the anterior 

 end and rounded posteriorly (2/*), a very short middle piece (1 p) 

 and a comparatively long tail (40/^) (PI. II., Fig. 25). 



At what age individuals discharge their sexual elements I 

 could not determine. It is certain that Lingula of about 30 mm. 

 in shell length discharge eggs, as I have observed the act. As 

 to the male, the spermatozoa are met with even in so young an 

 individual as one 7 mm. in length. Whether spermatozoa which 

 are so precociously developed are retained in the body for a 

 long time or are soon discharged, is quite unknown. 



