192 HUGH WATSON. 



sexangula and A. parva less than 4 mm. in front of the 

 diaphragm, and only a very short distance further forwards 

 in the other two species (PI. IX, figs. 28, 30, 31). On the 

 other hand, in A. gibbonsi and A. dimidia it is broader 

 and very much shorter, the exact position of its origin 

 depending on the length of the penis (figs. 27 and 29). 

 Probably the retractor originally arose in the neighbourhood 

 of the diaphragm ; but owing to the posterior position which 

 the diaphragm has come to occupy, the origin of the retractor 

 has moved forward. If this view be correct, the condition 

 found in A. sexangula and A. parva would be the most 

 primitive. 



The Spermatozoa. — PI. XXIII, fig. 151, shows the anterior 

 end of a spermatozoon of Ap era dimidia magnified 1200 

 diameters. It will be seen that the head is curved into the 

 shape of a hook, and is about "0035 mm. in length. The 

 '^ middle-piece," or proximal portion of the tail, is surrounded 

 by a spiral filament or flange, very loosely coiled, the Avhorls 

 being about "004.5 mm. apart. The posterior portion of the 

 tail is extremely long. In Apera sexangula and A. 

 gibbonsi rubella the head of the spermatozoon is straighter 

 and broader, though pointed in front. Fig. 150 shows the 

 anterior end of a spermatozoon of the latter variety. It will 

 be seen that in this form the whorls of the spiral filament are 

 a little nearer together than in A. dimidia, being separated 

 by a distance equal to the length of the head, namely, 

 •0037 mm. 



The spermatozoa of Testacella are very diiferent from 

 those of Apera, as may be seen from PI. XXIV, fig, 160. In 

 Testacella the head is globular, and there are two spiral 

 filaments surrounding the proximal portion of the tail. 

 Moreover the filaments are much more closely coiled than in 

 Apera, adjacent whorls being only about "0009 mm. apart 

 in T. maugei. In Rhytida capillacea {F<'r.) I find that 

 there are also two filaments, but they are not so closely coiled, 

 the whorls being separated by about "00165 mm. ; and in this 

 species, as well as in Natalina quekettiana (il/. & P.), the 



