BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 391 



nerve. In some pale species they are marked externally as two pinkish dorsal 

 streaks. These trunks, as already indicated, have a very different position from the 

 Ommatoplean nerves, being situated outside the circular muscular coat, and 

 between it and the great longitudinal. Two muscular coats (circular and internal 

 longitudinal) thus intervene between the nerves and the body-cavity and its con- 

 tents, whereas in Ommatoplea the nerves are within all the muscular layers. 

 In Meckelia annulata, the nerve-trunks are not placed as in Cerebratulus taenia, 

 which conforms to the Borlasian type, but lie between the external circular and 

 internal longitudinal muscular coats. This arrangement is characteristic of the 

 Meckelian type. 



In Cephalothrix, the peculiarity of the ganglia (as first pointed out by Prof. 

 Keferstein) is the advance of the almond-shaped upper lobes, so that the supe- 

 rior commissure is quite in front of the inferior (Plate XIII. fig. 1). The lateral 

 nerves are placed between an isolated longitudinal fasciculus and the great longi- 

 tudinal muscular coat of the worm. 



In regard to the innervation of the body by the lateral trunks, it is interesting to 

 observe the very long time during which detached fragments of the body survive in 

 several of the long Borlasians, such as Cerebratulus taenia, Dalyell, and the great 

 Lineus longissimus. A specimen of the latter, for instance, sent from St Andrews 

 in September, broke into pieces on the journey ; yet six months afterwards most 

 of the fragments were alive, although the sea-water had not been changed more 

 than once. The head and anterior portion of the worm, which scarcely measured 

 two inches at first, had now grown a body and tail that when crawling measured 

 at least seven inches, and of course capable of much greater extension, so that it 

 looked like an independent animal ; and this was accomplished without the aid 

 of any food, except perhaps what it might have acquired from the fragments of 

 its own body in the neighbourhood. Some of the latter measured about a foot in 

 length, and all lay coiled in various ways, with the ends puckered, and in most 

 cases fixed by a whitish cicatrix, which was firmer at one end than the other, 

 and occasionally tapered. A similar power of regeneration was observed in the 

 anterior end of Borlasia, Cerebratulus, Micrura, and Cephalothrix, when only a 

 fragment of the body was left behind the mouth ; and in Borlasia octoculata, a 

 very fragile species, reproduction of a complete head upon each of the fragments 

 ensues, if not with rapidity, at least with certainty* One of the most remark- 

 able features, to continue the case of L. longissimus as a type, was the gradual 

 development and elaboration of the products of the generative organs (in this 

 case the male elements) in the headless fragments, so that when in February 

 they were placed in clean sea-water, some gave exit to milky clouds of perfect 

 spermatozoa. This would seem in these animals to be the main aim of such a 

 provision, since their very length and softness, if not fragility, apparently court 



* Proced. Linn. Soc, June 1868. 



