652 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



the stomach, which is itself separated by a circular fold from the terminal portion 

 of the canal, the rectum ; from the stomach a number of branched radiating caeca 

 take their origin. Beneath the stomach is the large oblong central nervous system, 

 which gives off anteriorly a commissure surrounding the pharynx. The sexual 

 organs are hermaphroditic. The female organs consist of a number of ovarian 

 caeca, dorsal in position ; they open into the cloaca by a central uterine tube. The 

 testes are usually ventral in position, and ramify on either side of the stomach; 

 . the caeca unite on either side into a spermatic vesicle, which opens to the exterior 

 between the third parapodium and the margin of the body. 



In a few cases the radial arrangement of the body is considerably modified, 

 and it may even be entirely lost, the body becoming greatly lengthened and the 

 parapodia and suckers arranged in two parallel lines; while in StelecJwpus not 

 only has the external radial symmetry disappeared, but the muscular septa and the 

 muscles of the parapodia are no longer convergent. 



While usually the apertures of the body, as well as the parapodia and suckers, 

 are situated upon the ventral surface, in a few cases the oral and cloacal openings 

 are upon one surface, while the parapodia are upon the other. 



Broadly speaking, the myzostomes may be divided into two groups. In one 

 the body is stout and massive and of a solid consistency; the dorsal surface is 

 usually vaulted; as a rule, there are no cirri, or, if present, they are represented 

 by short inconspicuous processes. The ventral surface is bulged outward by the 

 strongly developed muscular mass, and from this central elevation a number of 

 smaller ridges run to the bases of the parapodia, and between them still smaller 

 ridges connect the central elevation with the suckers. On the dorsal surface there 

 are very often seen five pairs of feeble elevations, which mark the position of 

 the base of the hook apparatus, and which may become very much enlarged; 

 sometimes the whole intestine shows itself on the outside in the form of a series 

 of elevated ridges. 



In the other the body is thin, flat, membranous, and somewhat transparent, 

 with a more highly transparent marginal border, owing not merely to the lesser 

 development of the muscles, but also to the fact that the ramifications of the 

 intestines and genital organs do not quite extend to the periphery. The greater 

 delicacy of the body in this group is owing to the slighter development of the 

 muscles of the body, especially of the ventral muscular mass; but the presence of 

 a more transparent marginal border is not confined to these forms. This group 

 is also characterized by the possession of long cirri or cirrus-like processes on the 

 margin of the body, of which the number occasionally exceeds 20. 



There are numerous transitional forms which unite these two groups and can 

 with difficulty be assigned to either. Among these are the very remarkable types 

 distinguished by the possession of two, four, or six fingerlike caudal appendages. 



In the cysticolous myzostomes the individuals contained in a single cyst 

 either resemble each other in form and size or are very different. In the latter 

 case each individual is either male or female, and the two sexes are very unlike 

 in appearance, the female being usually from 50 to 100 times as large as the male. 



