238 NIERSTRASSIA FRAGILE. 



from the pericardial wall. In the first part of its course the aorta is of rela- 

 tively large size, but later becomes greatly compressed and difficult to follow 

 between the halves of the gonad. In the head region it loses its walls, the blood 

 entering sinuses that apparently have the usual relations. 



As in the case of the circulatory system the abundance of connective tissue 

 masks the course of the smaller trunks of the nervous system so that the 

 broader features only have been worked out. The brain presents the usual 

 characters, giving off laterally the pedal, lateral, and labiobuccal connectives, 

 and anteriorly, nerves which at once attach to groups of ganglion cells. As 

 noted in connection with the digestive tract these last named ganglia probably 

 correspond to those located about the bases of the cirri in other neomenians, 

 and in the present species they send off fibres that pass to the anterior section 

 of the alimentary canal whether it be an atrium or not. The pedal, lateral, and 

 labiobuccal ganglia and their connectives are normally placed, and as many 

 points throughout the body the first two are united by the usual commissures 

 and connectives. In the case of the labiobuccal ganglia the commissure was fol- 

 lowed posterior to the radula, but the abundance of muscle and connective- 

 tissue fibres makes it impossible to determine if there be other commissures or 

 a subradular system. 



In the posterior end of the body the pedal ganglia gradually diminish in size 

 and finally disappear from view. Almost to the end of their course they con- 

 tinue to be united by connectives with the lateral ganglia, but these show no 

 unusual development and the pedal ganglia lack the posterior enlargements 

 characteristic of some neomenians. The lateral gangUa, on the other hand, 

 terminate in globular masses, in the neighborhood of the pericardial-coelomoduct 

 openings, that are united by a conunissure passing dorsal to the rectum where 

 the latter unites with the more expanded section of the gut. 



The paired gonad, containing both ova and spermatozoa, extends as usual, 

 from about the level of the anterior end of the foot to the pericardium. In 

 both specimens the ova appear to be somewhat immature while the spermatozoa 

 are in all stages of development and especially in the mid section of the gland 

 are so numerous that they distend its walls to a considerable degree. Posteriorly 

 the halves of the organ gradually narrow, diverge sUghtly and communicate 

 with the small pericardium. 



The opening of each gonoduct is borne on the summit of a small papilla 

 (Plate 6, fig. 5) on the postero-lateral walls of the pericardium, and leads into 

 a tube which passes laterally and then posteriorly to unite with the ventral 



