DEPOSITION OF GENERATIVE ELEMENTS. 89 



make out the correct anatomy of the parts and the physiology of the process. Dr. Williams 

 states that the " segmental organs" in Lineus, Borlasia, and Nemertes correspond in number with 

 the transverse divisions of his great "alimentary caecum" (digestive cavity), and that there is only 

 one British species {Folia quadrioculata) in which it is possible to demonstrate the segmental 

 organs in situ as transparent objects. It is almost unnecessary to contradict the last statement, 

 since small specimens of most Nemerteans are more or less translucent. This author also 

 maintains that the group agrees in the structure of its generative organs with the type of the 

 lateral ovarian pouches of the Hirudinei, differing from the latter, however, in having the sexes 

 separate. Van Beneden and Keferstein give a correct account of the position of the ovaries 

 and sperm-sacs in the species examined by them ; but the term " biliary caeca" used by the 

 former is objectionable, as tending to confound the generative and digestive systems. M. de 

 Quatrefages imagined that the ova were extruded by a temporary aperture in the Enopla, and 

 pointed out that CErsted and Duges were wrong in averring that they escaped through the walls of 

 the body. (Ersted's observation, however, is correct, as subsequently proved by Van Beneden 

 and Keferstein. Frey and Leuckart erroneously conjectured that the ripe ova were shed from 

 the posterior end of the body, "as in Arenicola." 



12. Phenomena of the Deposition of Ova and Spermah 



ozoa. 



The ova and spermatozoa in A. lactifloreus would seem to attain full development in 

 February, March, and April ; but the breeding season of other examples of the Enopla ranges 

 from the latter month to November. Specimens of A. lactifloreus, which had been in confine- 

 ment seven months, deposited their ova about the middle of February, and wild examples a 

 little later. The actual number of ova was not counted ; but in one instance those from a single 

 female covered a circular space more than half an inch in diameter. In several species, e.g. 

 N. gracilis, N. Neesii, and A. spectadilis, the number of ova is immense. Occasionally, in a 

 crowded vessel, the ova of A. lactifloreus are found above the water-line, adhering to the vessel in 

 an irregular mass ; but they are held together only by accidental mucus, and easily fall asunder. 

 There is, therefore, a characteristic difference in regard to the deposition of the ova between this 

 group and the Anopla ; for in the latter they have a totally different shape, and a special invest- 

 ment of tough mucus. The only exception, so far as I have yet found, in regard to the deposition 

 of the ova in a free condition, occurs in the aberrant Nemertes carcinophila. The body of the worm 

 considerably diminishes after spawning, and assumes a flattened form, especially in large examples. 

 That impregnation of the ova (in A. lactifloreus) takes place only after deposition is proved by 

 segregating a female ready to spawn, for then it is found that no further change ensues in the 

 egg. Hence the large size of the male organs, as in fishes and other animals that shed their 

 secretion in the surrounding water. 



When fully developed, the mode of depositing the ova and spermatozoa may be illustrated 

 by the following account : — Two specimens, male and female, of N. gracilis were taken from a 

 deep vessel, and subjected to examination in a large glass trough. A very few minutes after 

 the male had been placed on the bottom of the cell tiny jets or jet-like wreaths of sperm- 

 fluid issued from the sides of the body, rather past the middle, and gradually increased in 

 number, both in front and behind. The animal was soon enveloped in a wavy cloud of the 



12 



