84 CEYLON PEARL OYSTER REPORT. 



which is about 2 to 2 '5 times the length of the head. This region terminates, as in 

 Tetrarhynchus herdmani, in a well-marked collar with somewhat scalloped edge. The 

 collar hangs back and overlaps the body region. 



The divisions between the proglottides are anteriorly very insignificant, but they 

 soon become distinct and the proglottides become a little longer. The total number 

 is between one and two hundred. But they are never very long, never even square. 

 The posterior proglottides are always some six or seven times as long as they are 

 broad, and the anterior perhaps twice as much again. Their edges are rounded, there 

 is no trace of overlapping, and in the latter half of the body the reproductive organs 

 cause an opaque patch in each segment. 



The diagnosis of Tetrarhynchus equidentatus is : 



Very small head, muscular proboscis sheaths reach anterior end of head ; unseg- 

 mented region, terminating in a well-marked collar, follows head ; proglottides, 100 to 

 200 in number, always much broader than long, rounded edges, no overlapping. 

 Proboscis hooks same size throughout, arranged in regular obliquely placed rows. 



Habitat : Intestine of Trygon ivalga, MULL, and HENLE. 



Tetrarhynchus herdmani, n. sp. Plate VI., figs. 108 and 109. 



The second species to Tetrarhynchus , found in the alimentary canal of Trygon 

 walga, and later in the same position in Rhynchobatus djeddensis, is a long and com- 

 paratively slender one. We had only three or four specimens, which averaged about 

 30 millims. in length. The head is small, about 1 millim. in length. It has two well- 

 developed lappets which, as usual, are very contractile and extensile. The four 

 proboscides emerge from very short muscular sheaths, which lie near the posterior 

 limit of the head. Instead of being half as long as the head, as is often the case in 

 the Tetrarhynchidae, they are perhaps one- twelfth to one -tenth the head length. The 

 proboscides which emerge from them are slender and covered with minute teeth, all 

 of the same size, arranged in spiral rows. The teeth are about O'Ol millim. in 

 length. 



The most characteristic feature of this Cestode, but one which it shares with 

 T. equidentatus, to be described is a peculiar fold or collar which hangs back from the 

 head and covers the anterior part of the neck. This collar seems to be very extensile. 

 In the figure drawn from the live specimen, its border of free edge is scalloped, but in 

 the specimens in spirit the collar seems more retracted and the free edge is smooth 

 and undivided. 



The neck is very short. Almost immediately after the head the proglottides are 

 indicated by sharp lines. There are some 80 to 100 proglottides present, all separated 

 from one another by clear, horizontal, and in no case concave, lines. Till the 

 proglottides become packed with eggs, the lateral contours are also straight and 

 parallel ; there is no overlapping. Thus the Cestode does not increase in width until 

 we get to the posterior proglottides, and in these the presence of the eggs entails a 



