58 CHAETODEliMA ATTEXr'ATA 



ill the ventral gill retractors and in this position may he traced almost to the 

 apex of the gill. Between its point of origin and its attachment to the rectum 

 at least four small nerves arise and extend fan-like into the ventral gill retractors 

 which they probably innervate. I have been unable to find any sulirectal 

 commissure. 



From the dorsal side of the suprarectal commissure four nerves arise, of 

 which the outermost ])air extends dorsally through the superior gill retractors, 

 and imbedded in the dorsal cloacal wall, whicli it jirobably inner\ates, may be 

 followed i'<ir a very considerable distance. The iimer pair jiursues much the 

 same route at first, but upon emerging from the doisal retractors and while 

 imbedded in the cloacal wall each nerve turns sharply upon itself, and bending 

 slightly toward the mid line and somewhat ventrally it enters the dorsal gill 

 retractor and in this position may be followed close to the tip of the gill. Each 

 of the branchia thus has a double nerve supply as in the ctenidia of the Chitons 

 for example. 



The gonad, with the usual characteristics, opens into the ])ericanliuni l)y 

 means of very short doi-so-\'entrall>' comjiressed tulies sejiai'ated l\y the aorta. 

 The pericardium is of umisual size, extending behind the heart nearly to the 

 posterior end of the l)ody. As may he seen Plate 36, tig. 2, it is interrupted l)y 

 the dor.sal gill retractors, but behind these muscles the cavity again becomes 

 continuous across the mid line, extending down the sides of the cloacal cavity 

 (Plate 25, fig. 5) and posteriorly forming a horn-like extension in the mid line. 

 The heart is the usual tubular organ but posteriorly it unites with an atrium, 

 which may be considered an auricle or an invagination of the ventral pericardial 

 wall continuous jiosteriorly with the efferent bi'anchial sinus. 



The openings of the coelomotlucts hold the usual position, at the sides of 

 the supi'arectal connnissui'c, but the tubes with which th(\v comnumicate are in 

 the first ]iart of their course very slender, ciliated, and somewhat convoluted. 

 In this condition they extend ventrally and join the glandular portion (Plate 36, 

 fig. 2). The cells of this .secretory portion are of the usual type, almost cubical 

 vacuolated elements containing a small concrcment. The jiosition of the 

 external opening is shown (Plate 25, fig. o). 



Wiren ('92) has accurately described a patch of glandular e])ithelium, a 

 modification of the cloacal wall, which on each side of the body suridunds the 

 openings of the gonoducts and extends to a certain extent ovci' the base of the 

 gills. The cells composing it are high and consist of very slender supporting 

 cells and glandular elements filled with an almost homogeneous substance, con- 



