DISTRIBUTION OF THE NEMERTEANS. 151 



There is a considerable difference in regard to the reproductive organs, for the Leeches are 

 hermaphrodite, whereas the Nemerteans are chiefly unisexual. The capsule of mucus for the ova 

 in Lineus is homologous with the cocoon of the leech ; the latter being apparently due to the 

 same abundant secretion poured forth by the general cutaneous surface, and is not necessarily 

 connected in any way with the numerous segmental organs. Some of the higher Annelids, 

 again, agree with the Nemerteans in discharging the generative products through lateral pores, 

 e.g. Harmothoe and Fhgllodoce. As in the Enopla, no metamorphosis occurs in the embryo of 

 the leech. All the latter are oviparous, whereas some of the former are ovo- viviparous. 



There is no feature to connect them with the Brachiopoda, which Mr. Morse 1 thinks should 

 be classed with the true Annelida; indeed, we are not prepared at present to admit the 

 relationship until we are more acquainted with the grounds on which the American author bases 

 his conclusions. 



I would be inclined to place the Turbellaria next the true Annelida, without the usual 

 interpolation of the Rotatoria. 



General Distribution of the Nemerteans. 



The Nemerteans have a very wide geographical range, extending from the arctic seas to 

 those of the equator, and it is probable they occur on every suitable sea-beach, as well as in the 

 surrounding depths. The forms adapted for swimming generally frequent the latter, and perhaps 

 only approach the shallow water at the extreme limit of their range, and in a somewhat modified 

 form, especially as regards size. Moreover, examples of the two great types (Enopla and Anopla) 

 are common both to the arctic seas and the antipodes. The range of the freshwater species is 

 involved in obscurity ; they have been found in various parts of the world, but not yet in this 

 country. 



With regard to the distribution of the British species, some forms are cosmopolitan, such as 

 Anvphiporus lactifloreus, Tetrastemma melanocepkala, Candida, and dorsalis, Nemertes JSfeesii, 

 Lineus marinus, L. gesserensis, Cerebratulus angulatus, and Carinella annulata, extending from 

 the Zetlandic seas along both eastern and western shores to the Channel Islands, and, in 

 addition, radiating widely all round. Thus I have received A. lactifioreus from Greenland, and 

 apparently the same form is described by M. de Quatrefages from the Mediterranean. Tetra- 

 stemma melanocephala, T. Candida, and T. dorsalis range from the latter to the extreme north of 

 Europe. Lineus gesserensis, again, appears to be even more widely distributed, for besides being 

 prevalent in the European seas, it (or a form almost identical in every respect) extends to the shores 

 of the United States. Cerebratulus angulatus attains greater dimensions in the seas of Greenland 

 and the Boreal province generally than it does in the Channel Islands. In other forms, however, 

 e. g. Lineus marinus, I have observed no apparent difference in bulk between those from Shetland 

 and those from Guernsey ; though at the same time it must be stated that nowhere have the 

 Nemerteans occurred of greater size and beauty than amongst the sheltered tangle-forests of the 



i l Ann. Nat. Hist./ 4 ser., vol. vi, p. 267. 



