BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 365 



the end of March proves. The actual number of ova was not counted ; but in one 

 instance the ova of a single specimen covered a circular space of more than half 

 an inch in diameter. Occasionally, in a crowded vessel, they are found above 

 the water-line, adhering to the glass in an irregular mass; but they are not con- 

 nected together by other than accidental mucus, and easily fall asunder. There 

 is, therefore, a marked difference in regard to the deposition of the ova between 

 this group and Borlasia ; for in the latter they have a totally different shape, and 

 a special investment 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 form Polia involuta, Van Ben. The bulk of the worm considerably 

 diminishes after spawning, and the body assumes a flattened form, especially 

 marked in large examples. That impregnation of the ova (in 0. alba) 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 into the 

 surrounding water. 



It is a mistake to describe, as Dr Johnston, M. de Quatrefages, and Drs 

 Frey and Leuckart have done, the ova as occurring in a free condition between 

 the body- wall, and the Barm or digestive cavity. They are always contained in 

 ovisacs. M. de Quatrefages observes that he found at the reproductive season a 

 milky liquid, containing corpuscles of conglomerated globules, in the generative 

 caeca ; and the succeeding descriptions and illustrations make it clear, as already 

 stated, that he refers to the walls of the digestive cavity, and the special elements 

 contained therein. Thus it is no wonder he had some difficulty in distinguishing 

 the sexes in the early condition of the generative products, since the cells would 

 be identical in every specimen. He indeed gives a tolerable figure of a cell from 

 the wall of the digestive cavity, as one of the true stages in the growth of the 

 spermatozoa;* and again refers (Plate XXII. fig. 2) to the glandular wall of the 

 said cavity as representing generative cseca. The spermatozoa, therefore, which 

 he shows, had either been discharged externally, or procured from a specimen in 

 such a condition as to leave no room for doubt. His figure f of the spermatozoa of 

 iV. balmea is incorrect, for the body is too short and thick. He considered that it 

 was only after the granular corpuscles fell out of the cseca into the lateral cavities 

 that they assumed their special characteristics as sperm-cells. He thus failed to 

 make out the correct anatomy of the parts and the physiology of the process. Br 

 Williams "I 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 (Polia 

 quadrioculata) in which it is possible to demonstrate the segmental organs in situ 



* Op. tit. pi. xxi. fig. 4, and still more plainly in pi. xxii. fig. 2. 



t Op. ait. pi. xix. fig. 6. + Philos. Transact. 1858, p. 131. 



VOL. XXV. PART II. 5 A 



