KEMARKAüLE srEClEÖ OF ECIIIUKOIDS. üo 



The segmental organs present some very remarkable points, 

 though, taken singly, they are of the ordinary type of structure. 

 Each organ represents a conspicuous sac of an elongate club-like 

 shape, tapering towards tlie inner end. It may be o.5 cm. long 

 and about 8 mm. broad at the bulbous base. The internal 

 opening, present at the base, is provided ^Yith two, relatively 

 short, sj)iral lobes. 



The organs are present in a rather numerous number, though 

 in a far less number than in Th. lœnioides. Moreover, the 

 number varies with individuals and presumably also with the 

 sex. In a general way I may say that it ranges from 13 to 27. 

 The organs are by no means indefinite as to the mode of occur- 

 rence, but show a decided tendency towards a symmetrical and 

 segmentally jmired arrangement. Six or seven pairs of them 

 may be distinguished, opening ventrally on both sides of the nerve- 

 cord. The pairs are well separated and lie equidistant from one 

 another, the first pair occurring only a short distance behind 

 the mouth and the last pair behind the middle of the body. 

 But it must be mentioned at once that the pairs are not always 

 perfect nor strictly symmetrical with respect to the number of 

 segmental organs composing each half-pair on either side of the 

 ventral nerve-cord. Thus, while in some cases a pair consists of 

 two segmental organs — one on each side of the body — and is thus 

 perfect, in other cases a half-pair may consist of a group of l-o 

 segmental organs, the number in each group varying in different 

 pairs and very frequently also on the two sides of one and 

 the same pair, which is thus rendered to consist of two sym- 

 metrical or assymetrical groups of the organs. It may even happen 

 that a single organ exists on one side, but none on the other to 

 complete a pair. Where more than one segmental organs occur in 



