154 



DR. n. LYSTER JAMESON ON THE 



[Mar. 4, 



connective tissue of the mantle-margin, where it is attached to 

 the "pallial line," especially along the ventral border, and 

 around the insertion of the siphonal musculature. The favourite 

 place seems to be the dorsal side of the latter. The individual 

 Sporocysts are embedded in and closely adherent to the bundles 

 of muscle-fibres. 



In young examples I found small, simple, spherical or oval 

 Sporocysts, about '5 mm, in diameter, and containing 6-10 

 Oercarise, but in larger examples groups of Sporocysts occur 

 (PI. XYI. fig. 9, and text-fig. 24). 



Text-fig. 24. 



Tapes decussatus. 



Group of Secondary Sporocysts, as seen in a pressure preparation. 



m.t., muscular tissue of Tapes ; sp.c, Sporocyst ; cer., Cercaria contained 

 in Sporocyst. 



The individual Sporocysts in these cases are often larger than 

 the simple ones. The groups are, from their position, their 

 relations to one another, and their progressive increase in size 

 and number of constituent cysts as Tapes grows older, evidently 

 produced by budding or secondary division of the original simple 

 ones. Their growth is very slow, and their duration of life must 

 be practically co-extensive with that of their host. In large 

 specimens of Tapes they appear to represent several successive 

 infections ; as we may have, in the same individual, large groups, 

 probably several years old, smaller ones containing three or four 

 cysts, and finally little single Sporocysts like those found in young 

 individuals. On the other hand, in young Tapes, 10-20 mm. 

 long, although the simple cysts and small groups occvir, the large 

 groups are not to be fovmd. 



Specimens of Tapes measuring 12x 10 mm., with one marked 

 period or interruption in the growth-rings on the shell, contained 



