578 



OLIGOCHAETA 



The structure of the spermathecae in the Eudrilidae forms also a very interesting illustration of 

 the 'substitution of organs.' The group is distinguished, as has been shown, by the important fact 

 that the organs, which, largely at any rate, serve the purpose of storing up sperm received from 

 another individual, are coelomic sacs, not, as in all other Oligochaeta, invaginations of the 

 epidermis. There is, however, some evidence which goes to prove that these sacs have gradually 

 taken the place of organs constructed on the plan which characterizes the vast majority of the 

 Oligochaeta. The various genera of Eudrilidae can be arranged in a series which illustrates this 

 gradual substitution The first term in the series is, perhaps, represented by the genus Heliodrilus ; 

 here there is a long spermatheca which reaches from the eleventh segment, where the external 

 pore is placed, as far back as the thirteenth. The spermatheca is expanded at the blind 

 extremity, and the extreme tip is again narrowed ; the very end of the sac is enclosed in 

 a sac of peritoneum, which is itself enclosed in a larger sac, communicating with the ovarian 



sac, &c. ; in sections the columnar epi- 



lig. 48. thelium lining the spermatheca can be 



easily distinguished from the peritoneal 

 cells lining the sacs which partly enclose 

 it. This, however, is the only reason which 

 leads to the inference that the organ which 

 I have called the spermatheca is really 

 strictly comparable to the spermathecae 

 of other Oligochaeta. It might be con- 

 sidered a proof that the spermatheca lies 

 in a coelomic sac ; but the fact that there 

 are two sacs of that nature, one enclosing 

 the other, shows that it is necessary to he 

 careful in drawing such an inference. In the 

 allied genus Hyperiodrilus (figs. 47 and 48) the 

 spermatheca is not evident on a dissection 

 of the worm ; it is a small elongated sac 

 which lies in the interior of a large sac 

 which surrounds the gut, and is prolonged 

 backwards above the gut as a pouch of oval 

 contour and considerable size ; on a dissec- 

 tion it appeared as if it was this large 

 sac which opened directly on to the exterior ; but this appearance is shown to be due to the fact 

 that the sac in question closely invests the real spermatheca up to the very point of opening of the 

 latter. In the genus Hyperiodrilus, therefore, there is a great reduction of the spermatheca, coupled 

 with a great increase of the coelomic sac which surrounds it. So many of the genera of Eudrilidae 

 are imperfectly known, as regards their minute anatomy, that a stage immediately following this 

 ig not at present forthcoming ; the nearest appears to me to be Paradrilus ; in this genus, as ROSA 

 has pointed out (35), the spermathecal sac is not an invagination from the exterior, but is 

 a coelomic sac, which communicates with the exterior by a narrow tube, the epithelium lining 

 which is manifestly epidermic in origin ; in transverse sections the abrupt change in the character 

 of the cells can be seen ; I infer that the epidermis-invagination is the remnant of the spermatheca 

 of such forms as Hyperiodrilus and Heliodrilus. In Eudrituiilm there is a still lower, or, rather, 

 a more specialized, condition ; in this genus, at any rate in two species, E. cotterilli and E. bmnneus, 



TRANSVERSE SECTION THROUGH SEGMENT XIII 

 OF HYPERIODHILUS. 



i. Spennathecal sac. 2. Oesophagus. 3. Calciferous glands. 

 4. Body-cavity. 5. Ovary. 6. Spermatkecal pore. 7. Nerve- 

 cord. 8. Sac into which oviduct (9) opens. 



