THE NEMERTINI 161 



or excitant organ. Its true nature, and the fact that it is quite distinct 

 from the gut, was first recognised by Frey and Leuckart in their account 

 of Linens gesserensis (1847), where, too, the real character of the mouth, 

 hitherto regarded as the genital pore, was established. 



The vascular system was recognised as a closed system of canals by 

 Quatrefages (36) ; and our present knowledge of the comparative anatomy 

 of the system is due to Oudemans (35), who in the same memoir gave an 

 account of the excretory system in a variety of genera. This excretory 

 system was originally described by Max Schultze in 1851 as a branched, 

 ciliated canal ; but it remained for Burger (7) to make the valuable dis- 

 covery that these canals terminate blindly in a multicellular swelling, 

 containing a flame, which had, however, already been seen by Silliman 

 in 1885 in Tetrastemma aquarum dulcium, and was figured by Dendy in 

 1890 for Geon. australiensis (10). 



The genital sacs were already known to Oersted, and the fact that 

 the worms are dioecious was recognised by Quatrefages, and was a reason 

 for separating the Nemertines from the Turbellaria. The nervous 

 system was correctly described by Duges (1828), who fell into the curious 

 error of regarding the brain as a " heart," and the lateral nerves as 

 " blood-vessels," owing to the fact that the system is tinged with red in 

 the larger forms. Our present knowledge of this system is due to 

 M'Intosh and to Hubrecht (17), while the finer histology of the system 

 has been studied by Haller (16), and by Burger (6). 



The cephalic fissures of the larger genera, such as Linens and Cere- 

 bratulus, early attracted notice, but the function of the peculiar cerebral 

 organs associated with them has been variously interpreted. Both Delle 

 Chiaje and Hubrecht maintained that they were respiratory, while van 

 Beneden took them to be excretory. The detailed account of these organs 

 which we are now able to give rests upon the work of Dewoletzky (11) and 

 of Burger (5). 



But although the study of anatomy and histology has made much progress 

 in these recent years, the embryology of the class still presents a wide field 

 for future investigation. The Pilidium larva, originally described in 1847 

 by Joh. Muller (33), has been studied by several zoologists of note, especially 

 by Metschnikoff (28), and by Butschli/ and by Burger. The larva of 

 Desor has been the subject of research by Barrois (1), and by Hubrecht 

 (19), while the direct development of Cephalothrix has been studied by 

 Dieck (12), and that of various other genera, quite recently, by Lebedinsky 

 (26). 



With regard to the taxonomy of the Xemertines, reference need only 

 be made to three zoologists : Max Schultze (1853) divided them into two 

 orders the Anopla and Enopla. This classification was generally adopted, 

 till Hubrecht put forward his threefold division into Palaeonemertini, 

 Schizonemertini, and Hoplonemertini, depending on the superficial position 

 and diffuse condition of the nervous system in the first, on the cephalic 

 slits in the second, and the armed proboscis in the third order. More 

 recently Biirger (8), laying stress on the relative depth of the nervous 

 system, and on the structure of the body wall, proposed the four-fold 

 division adopted in the present article. 



ii 



