BY PROFESSOR AV. A. HASWELL. 505 



mochte, den Schwanz elastisch zu machen und ihm einen gewissen 

 Grad von Rigiditat zu verleihen. Zwischen Muskeln und Achsen- 

 strang liegt eine Schicht von grossen platten Zellen, die bei 

 Cercaria macrocerca bei welcher der Achsenstrang zu eines 

 machtigen Entwickelung heranwiichst, sehr sonderbar veriistelte 

 Formen haben." 



Here it will be observed the axial strand is described as being 

 composed of cells, which have become converted into a hyaline 

 connective tissue acting as the main support of the organ. 



The structure of the tail in Bucephalus is described b}'' Ziegler 

 (18), but as this is by no means a typical Cercaria the statements 

 which he makes probably do not hold good generally; they are 

 certainly entirely inapplicable to the form Avhich I am now 

 describing. 



Thiry (13) states that in Cercaria macrocerca the tail has an 

 external circular layer of muscle. The longitudinal fibres run in 

 two broad bands at the sides. Beneath this is a mass which 

 anteriorly consists of large cells, posteriorly appears homogeneous. 



Schwarze's account (12) of the structure of the tail in Cercaria 

 armata is also not in any way in agreement with what is to be 

 observed in the Cercaria from Mytilus. He describes the con- 

 tractile substance of the tail as forming an axial strand surrounded 

 by a layer of cj'-stogenic cells, and these again by a further layer 

 of contractile substance, the outer layer being connected with the 

 axial strand by numerous radiating bundles. 



Reuss (11, p. 378) states that in the Sporocyst of Distomum 

 duplicatum the tail has a layer of cells underlying the cuticle, 

 and the interior is traversed by transverse muscular fibres. 



It does not seem to be possible to reconcile these various state- 

 ments, and, if they are correct, there must be considerable diversity 

 in the structure of the tail in different Cercariae. 



In the posterior part of the body opening near the root of the 

 tail is the excretory bladder. This is broad behind, narrower in 

 front, where it becomes forked. Each of the branches terminates 

 anteriorly in a slight enlargement into which opens the corres- 



