324 Transactio s.—Z "logy. 



teeth of which are, however, very long. These bristles are held 

 together by fine transverse connections, and when in action 

 during life are kept in the one plane and moved as a single 

 structure, but in preserved specimens they become separated and 

 spread out towards the distal end. The tail is also capable of 

 considerable contraction and expansion, and its proportionate 

 length and width accordingly varies greatly, but in the condition 

 most usually maintained the width is about one-eighth of the 

 length. 



The head is usually more or less rounded when the Cercaria 

 is swimming freely, but in addition to this mode of motion the 

 Cercaria can creep along by means of the worm-like motions of 

 the head caused by contraction and expansion, and accordingly 

 the head then assumes other shapes and may be much elongated. 

 In iig. 2 the anterior portion is shown in the act of contracting 

 and consequently widening, while the posterior portion is still 

 elongated. The general colour of the whole Cercaria is whit Is i, 

 semitranslucent, but on the head there are usually a number of 

 small irregular pigment-spots of a light-brown colour. 



The general structure of the Cercaria appears to present the 

 usual features : the buccal sucker is situated at the anterior ex- 

 tremity, and is the foremost portion when that part of the body 

 is being extended ; the posterior sucker is about equal in size to 

 the anterior and is situated a little anterior to the middle of the 

 head ; in iig. 2 it appears rather nearer the anterior end owing 

 to the contraction of that portion of the head. There is a well 

 marked cuticle, and around the anterior sucker this hears a num- 

 ber of fine short seta 3 . The internal organs are well developed ; 

 during life the most conspicuous is a large V-shaped structure 

 extending from the posterior extremity of the head nearly to the 

 anterior end, and presumably representing the contractile bladder 

 of the excretory system ; in life this is darker than the rest of the 

 head, and is tilled with minute granules. The central portion of 

 the tail presents much the same appearance, bul whether the 

 bladder actually extends into the tail or not I am not certain. 



The alimentary system is also well developed and of the usual 

 structure; the pharynx is small, nearly circular, and situated 

 close up to the buccal sucker; the oesophagus narrow and short. 

 and the two lateral branches of the intestine are also narrow. 



The reproductive organs also appeal to lie well developed in 

 most of the Cercarice examined, and in one which was accidentally 

 ruptured while under examination I saw mature spermatozoa; 

 in some the genital aperture could be seen about midway between 

 the two suckers, and connected with it an oval structure extending 

 back as far as the posterior sucker and apparently representing 

 the. rudiment of the genital atrium. I have, however, not, 



