THE ANATOMY. COCOON 145 



by EISEN ' tubercula pubertatis.' These are swellings on some of the segments which 

 form part of the clitellum : they appear, however, before the clitellum is developed ; 

 their structure is exactly like that of the clitellum, except that there are more 

 unmodified hypodermic cells among them ; they are rather prominent, extending beyond 

 the general body-surface, and they afford useful specific characters by their position 

 and number; in a few forms, for example in the genus AUuriis, the papillae form 

 a continuous raised band, traversing all the segments upon which it is developed 

 without a break ; it is more usual, however, for the tubercula to be paired, 

 each pair being limited to a single segment. These organs differ morphologically 

 from the papillae of the Perichaetidae, in that they are not associated with glands 

 lying in the body-cavity ; it is possible, however, that the difference is not one of 

 first-rate importance : for the glands of the Perichaetidae are merely masses of 

 glandular cells from the epidermis, which have got withdrawn into the body-cavity, 

 but which correspond to the glandular cells which are found upon the tubercula 

 pubertatis. I have dealt with the glands of a tubular character, which FRAISSE 

 has described as opening on to the exterior on the tubercula, in connexion with the 

 formation of the spermatophores. 



In various Acanthodrilidae there are the papillae already spoken of as having 

 been termed ' Wollustorgane ' by MICHAELSEN; these have much the appearance of 

 the papillae in certain Perichaetidae, but are found, on a microscopic examination, 

 to be composed of elongated epidermic cells, and have in all probability rather 

 a sensory than a glandular function ; they are often valuable marks for the 

 discrimination of species ; I have made use of them, for example, in distinguishing 

 the two allied species Acanthodrilus georgianus and A. aquarum dulcium; their 

 minute structure was first made known in the former species by MICHAELSEN. 

 These papillae are very similar to papillae which I have described in the Phreoryctid 

 Pelodrilus. 



14. The Cocoon. 



All Oligochaeta, without so far as is known an exception, form cocoons in 

 which the ova and the sperm * are deposited, with or without albumen, set apart 

 from the nourishment of the embryos. The cocoon consists of a chitinous substance, 

 and it is formed by the activity of the clitellum ; that this is the case has been 

 proved by direct observation ; thus VEJDOVSKY (9) watched the secretion of the 

 cocoon in Rhynchelmis from the clitellum. As all Oligochaeta possess this organ at 

 the time when the genital products are ripe, there is no reasonable doubt but that 



1 It does not appear certain that sperm always exists in the freshly-deposited cocoons. 



U 



