MOLGULA. 37 



or primary vessels is composed of a vast multitude of 

 secondary vessels ; those between the folds have an 

 arched or more or less sinuous course in the longi- 

 tudinal direction, but nevertheless have a tendency to 

 curve towards the base of the folds, forming in fact 

 the outer, irregular coils of large spirals. Within the 

 folds these incipient coils go to form a series of small 

 spirals usually arranged in pairs, composing the ulti- 

 mate marginal recesses of the pouches. The pouches 

 are limited by the large primary vessels, and are 

 divided into two by the smaller primary vessels and 

 then again each is subdivided ; so that each pouch is, 

 as it were, formed of four spirals arranged in pairs. 

 It is uncertain whether this arrangement is constant, 

 but it can usually be seen in well-preserved specimens. 

 Other small vessels ramify in a transverse direction, 

 but converge more or less regularly from the primary 

 vessels towards the spiral centres ; these are the 

 radiating vessels. 



Besides the large folds already described there are 

 numerous, narrow, longitudinal, membranous bands 

 which stretch the whole length of the organ ; they are 

 mostly confined to the folds, there being usually four 

 on each side, while between the folds there are occa- 

 sionally one or two. These minute membranous bands 

 are the homologues of the longitudinal bars of Ascidia ; 

 and like them are connected with the vascular sur- 

 face only at the points where they cross the large or 

 primary vessels. 



The above is the description of the branchial organ 

 as it usually appears in M. conchilega. In M. simplex 

 and M. complanata the spiral arrangement of the 

 secondary vessels is much more distinct, and is well 

 defined at the circumference of the coils. In these two 

 forms the spirals therefore seem large and occupy 

 nearly the whole surface, the centres terminating, 

 apparently, in the pouches of the branchial folds. 

 The minute membranous bands are in these two species 

 confined entirely to the folds, there being none between 



