BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 363 



Van Beneden, though he noticed the sac in Borlasia, does not mention more than 

 " fossettes cephaliques" in this group. 



Organs of Reproduction. — The sexes are separate, and the generative products 

 developed between the inner muscular layer of each lateral region of the body 

 and the glandular digestive chamber, and enclosed in special cavities (Plate XL 

 fig. 2) formed by transparent membranous sacs (e), which are connected with the 

 inner muscular layer of the body- wall. In the matured specimen the ova are 

 observed to extend from the oesophagus almost to the tip of the tail, each ovary 

 containing from one to seven ova, which, when fully developed, are seen with 

 the naked eye through the attenuated parietes of the body. They attain a 

 comparatively large size before leaving the body of the parent ; and it is curious 

 that they are not much less in bulk in small specimens, though few in number. 

 The female in the ripe state has a greyish- white appearance, with the dorsal tube 

 for the proboscis extending nearly from end to end, though its diameter is lessened 

 posteriorly from the encroachments of the ovaries. The sperm-sacs in the male 

 generally have a pyriform or flask- shaped aspect, especially in the early con- 

 dition, being attached to the body- wall by a narrow tubular neck, which at the 

 proper period doubtless gives transit to the contents of the sac. In the early 

 condition the latter is finely granular, then cellulo-granular ; and in the mature 

 state it has a finely fibrous or streaked appearance from the spermatozoa. Some- 

 times both granules and spermatozoa occur in the same sac, and then the former 

 are often observed to be somewhat regularly arranged (Plate VII. fig. 12). The 

 spermatozoa in 0. alba (Plate VIII. fig. 13) have a slight curve of the body, which 

 gently widens from the tip and ends in a perceptibly larger rounded knob, from 

 which the long tail proceeds. The mature males are easily distinguished from 

 the females by their whitish or pinkish aspect, and their bodies are less bulged. 

 The spermatozoa of O. gracilis (Plate IX. fig. 8) are most active wriggling struc- 

 tures, of a more slender shape than in O. alba or Tetrastemma (Plate VIII. fig. 14), 

 appearing under a power of 1000 diameters as simple rods, slightly larger 

 towards the end from which the elongated and very fine tail proceeds. The 

 sperm-sacs are very numerous in Polia involuta; but the tenuity of the sperma- 

 tozoa (Plate IX. fig. 9) renders their exact structure somewhat obscure. The 

 body of the spermatozoon is elongated, gently curved, and slightly thickened at 

 the end from which the tail proceeds. It is very common, moreover, to observe 

 one or more minute clear globules attached to the body of the spermatozoon, so 

 that the structure seems to have a tail at both ends, or a large flattened head. 

 These appearances have misled even so experienced an observer as M. Van 

 Beneden, who figures* these organs as possessed of a somewhat globular body, 

 with a tail at each pole. But, independently of the strange exception which 

 such a condition would make in Nemertean physiology, the frequent occurrence 



* Recherches, &-c. Mem. l'Acad. Belg. t. xxxii. pi. Hi. fig. 11. 



