58 CHAETODERMA ATTENUATA 



in the ventral gill retractors and in this position may be 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 subrectal 

 commissure. 



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

 which the outermost pair extends dorsally through the superior gill retractors, 

 and imbedded in the dorsal cloacal wall, which it probably innervates, may be 

 followed for a very considerable distance. The inner pair pursues nuich the 

 same route at first, but upon emerging from the dorsal 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 pericardium by 

 means of very short dorso-ventrally compressed tubes sejjarated by the aorta. 

 The pericardium is of unusual size, extending bchinil the heart nearly to the 

 posterior end of the body. As may be seen Plate 36, fig. 2, it is interrupted bj^ 

 the dorsal gill retractors, but behind these muscles the cavity again liecomes 

 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 maj' be considered an auricle or an invagination of the ventral pericardial 

 wall continuous posteriorly with the efferent branchial sinus. 



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

 the suprarectal commissure, but the tubes with which they conununicate are in 

 the first part of their course very slender, cihated, 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 concrement. The position of the 

 external opening is shown (Plate 25, fig. 5). 



Wiren ('92) has accurately described a patch of glandular epithelium, a 

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

 openings of the gonoducts and extends to a certain extent over the base of the 

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

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



