ATLANT. DEEP-SEA EXPED. 1910. VOL. III]. 



PELAGIC NEMERTEANS. 



17 



behind the brain some of the longitudinal fibres penetrate 

 the circular layer, so that these muscles in the greatest 

 part of the wall form a distinct layer within the longi- 

 tudinal musculature, without beeing interwowen with it as 

 is the case in many forms. 



As mentioned in the diagnosis of the family the 

 dorsal commissure of the vessels in the head is lacking 

 and the dorso-median vessel is never in connection with 

 the proboscis sheat and never enters the rhynchoccelomic 

 cavity. 



The longitudinal nerve-stems are placed close to the 

 lateral edges of the body. 



The testicles form an almost regular row in the head 

 on either side along the nerve-stems; the opening of 

 their ducts are placed lateroventrally. The ovaries are 

 not numerous as only 8 pairs could be counted, the 

 openings of which sometimes are lying outside the stems 

 of the vessel and nerve, sometimes between them and 

 sometimes to the inside of them. 



Habitat: 

 St. 80 (Lat. 47° 34' N; long. 43° 11' W) u h about 1666 



metres depth (2500 metres of wire). 

 St. 81 (Lat. 48° 2' N; long. 39° 55' W) V1 h about 1666 



metres depth (2500 metres of wire). 

 St. 88 (Lat. 45° 26' N; long 25° 45' W) 18 /v about 1333 



metres depth (2000 metres of wire). 



Pelagonemertidse (Moseley) Brinkmann emend. 

 Small or medium sized pelagic nemerteans. 

 The body usually applanated and fairly broad in 

 proportion to length. The caudal fin is, where pre- 

 sent, formed by lateral excrescences from the tail, 

 not by flattening of same. Mouth and proboscis 

 pore separate. Stomach, pyloric tube and intestinal 

 caecum much reduced. As a rule, the intestinal 

 diverticula of the body form a dorsal and a ventral 

 main branch. The musculature of the proboscis 

 sheath is composed of an inner longitudinal and 

 an outer circular layer. The bundles of muscle 

 fibres following the lateral nerves always de- 

 veloped. The dorso-median vessel rudimentary 

 or lacking. The testicles are found in two groups 

 at the fore end of the head. Rudimentary eyes 

 generally found. 



Natonemertes nov. gen. 

 i ody pointed at the hinder end, whithouttail 

 fin, Testicles in two groups behind the brain. 

 Intestinal caecum with one pair of diverticula. 



Natonemertes acutocaudata n. sp. 



(Plate II, Fig. 26). 



The "Michael Sars" expedition brought home a single 

 specimen of this interesting new genus, which forms a 



BRINKMANN — 3. 



connecting link between Peiidonemertes and forms like 

 Pelagonemertes and Balcenanemertes. 



Length 9 mm., greatest breadth 4 mm., greatest 

 thickness 2.25 mm. The dimensions are those of a com- 

 paratively small species: it is probable, however, from 

 the degree of development of the testicles, that the speci- 

 men in question had not yet reached its full size. 



Colour, in formalin, a pale pink. 



The surface epithelium, of which some portions were 

 preserved, distinctly showed the bulb-shaped sensory 

 organs known from Neetonetnertes. 



The muscle-layers of the body wall greatly reduced, 

 and laterally almost altogether lacking. 



Mouth and proboscis pore separate. The stomach 

 very short, passing over already in the region of the 

 brain into a short pyloric tube. The intestine furnished 

 with 15 — 20 pairs of large, slightly ramified diverticula, 

 showing, where best developed, some trace of ventral 

 branches. The intestinal caecum is very short and only 

 provided with one pair of diverticula. 



The proboscis is long and powerful; the slightly 

 curved stylet base is armed with two rows of stylets. 12 

 proboscidial nerves are developed. The proboscis sheath 

 extends right out into the tail, terminating immediately 

 in front of the anus. The muscle bundles following the 

 lateral nerves are very thin. 



The dorso-median vessel ends blindly shortly after 

 passing through the walls of the proboscis sheath. 



The testicles are ovate, and form two groups with 

 four in each, close behind the brain. 



Habitat: St. 101 (Lat. 57° 41' N; long 11° 48' W) 

 6 /s, at 1333 metres depth (2000 metres of wire out). 



Balxnanemertes Blirger (1907) 1912. 



Small species. The caudal fin strongly devel- 

 oped. Male and female with short lateral ten- 

 tacles one on either side of the head. The 

 testicles lie close together in two groups beside 

 or in front of the brain. 



Balxnanemertes lobata Joubin 1906. 

 Syn. Neetonetnertes lobata Joubin 1906 (13). 



This species is hitherto known only in Joubin's 

 type specimen, the description of which is unfortunately 

 altogether superficial. Only the large quantity of material 

 available - - of which one specimen from the present 

 expedition — rendered it possible to identify it at all. 



The specimen in question was slightly larger than 

 the type. Length 7 mm., greatest breadth 2.5 mm., 

 greatest thickness 1.6 mm., length of tentacles 0.8 mm. 



No information is available as to the appearance of 

 the animal in a living state. 



