NERVOUS SYSTEM OF PELAGIC NEMEKTEANS. 137 



Pyloric nerves. In some species, at least, a pair of minute 

 nerves, originating from the lateral nerve cords, enters the anterior 

 end of the pylorus. 



Caudal nerves. The posterior extremity of the body is sup- 

 plied with nerves in two different ways. In Proarmaucria the 

 posterior end of each of the lateral nerves divides into two 

 branches, the larger of which enters the posterior commissure, 

 while the smaller supplies the extremity of the body. In Ncurone- 

 mertes and some other forms the caudal extremity is supplied 

 with small nerves from the commissure itself (Fig. i). 



Sense-organs. Even the most primitive of the littoral nemer- 

 teans are provided with special sense-organs as differentiated por- 

 tions of the integument, and in the higher groups ocelli, cerebral 

 sense-organs, and frontal sense-organs are of frequent occurrence. 

 In addition, otocysts are found in a few species. But in the 

 pelagic forms no such organs have been found in any of the 

 species studied. Possibly vestiges of one of these types of sense- 

 organs were discovered by Burger ('09) in Balanancmertes. 

 These are considered by him to be rudimentary ocelli, and similar 

 organs were shortly afterward found by Brinkmann in Pelagone- 

 mertes rollestoni. Further studies have shown them to be widely 

 distributed among the Armaueriidce and Pelagonemertida. They 

 are of two distinct types, as follows : 



Subcutaneous sense-organs. In these forms a small cluster of 

 sense-organs, whatever their nature may be, is situated beneath the 

 basement layer of the anterior margin of the head, frequently on 

 either side of the rhynchodeal opening. Each of the end-organs 

 consists of a pear-shaped group of cells, connected with a branch 

 of one of the cephalic nerves (Fig. nB). Some of them lie in 

 contact with the inner border of the basement-layer, while others 

 may be situated somewhat deeper in the cephalic tissues, and in 

 or even beneath the muscular layers. Since they differ widely 

 from typical nemertean ocelli in structure, it seems quite possible 

 that these may be special sense-organs peculiar to the pelagic forms 

 rather than that they represent degenerate ocelli. 



Integumentary sense-organs. A second type of sense-organ 

 occurs in the integument of the head and less frequently in other 

 parts of the body in certain genera. These consist of compact 



