27 2 DISCOVERY REPORTS 



1*3 mm. respectively. The head is blunter than the tail and is marked off from the body 

 by lateral opposite furrows and a complete circular chevron groove whose apex points 

 back ventrally. No eyes are visible. The colour is usually bleached but may be yellowish. 



Anatomy. The epithelium is thick. In one series of sections it was at the head almost 

 twice as thick as the basement membrane, circular and longitudinal muscles together. 

 Farther back on the body its relative thickness was less. 

 I have some doubt of the value of comparative measure- 

 ments of the epithelium and muscle layers, for another 

 series shows a thick basement membrane not much 

 stained, and a clear layer of circular muscles, the two 

 together being about equal in thickness to the epi- 

 thelium; it seems probable that bad fixation may be 

 responsible for these differences. 



The head gland is compact. It opens by a median 

 pore at the tip of the head. There are three strands, one 

 median dorsal and two lying laterally along the rhyn- 

 chocoel. The former nearly reaches the dorsal com- 

 missure. The oesophagus opens into the rhynchodaeum 

 near the snout and enlarges to the stomach just in 

 advance of the brain. The anterior caecum sends forward 

 two diverticula above the lateral nerves which overlap 

 the posterior ends of the dorsal ganglia (Fig. 48), 

 although the relative position of the branches and ganglia 

 is altered by the state of contraction. The single ex- 

 cretory ducts open before the junction of the anterior boc j y 

 caecum with the gut. 



The proboscis is stout. The armature consists of a main stylet on a pear-shaped base 

 larger than itself and two accessory reservoirs each with one or two stylets. There are 

 ten nerves. 



The ventral ganglia are a little larger than the dorsal. The cerebral canals appear as 

 furrows on the ventral surface just in advance of the anterior pair of eyespots. They 

 deepen and sink in. The organs join the dorsal ganglia from a ventro-lateral direction at 

 the level of the ventral commissure (Fig. 48). 



Fig. 48. Tetrastemma georgiatmm, 

 Burger. Graphic reconstruction of 

 the head and anterior end of the 



Tetrastemma gulliveri, Burger, 1893 (Plate XVI, figs. 11, 17; Fig. 49). 



This species was originally described and sketched at different times under the desig- 

 nations N 10 and N 14, but intermediate forms showed later that the differences lay in 

 the size of the body and the distinctness of its markings. Subsequent investigation of 

 the armature proved the identity. Thirty-four animals were examined in life, and from 

 sections the following single specimen was sorted from the collection: St. 144, 5. i. 27, 

 NCS-T, 155 178 m. (N 123). The lengths varied from 4 to 17 mm., the breadths 

 from o-6 to i-6 mm. 



