No. 2.] NOTES ON AEOLOSOMA TENEBRARUM. 179 



The head segment, which is broader than the following 

 segments, is separated from them by a constriction, and seen 

 from above, it appears to have a ciliated pit on each side like 

 those of the Turbellaria. The mouth is on the ventral side 

 and is overhung by the prostomium, which is ciliated on its 

 lower surface, and serves as a tactile organ. There is no 

 proboscis. 



Intersegmental septa were not found, but the segmentation 

 is defined by the nephridia and the setae. Each segment has 

 one pair of nephridia and four bundles of setae, placed dorso- 

 and ventro-laterally. There is also a segmental arrangement 

 of single, nucleated muscle fibers which extend from the ali- 

 mentary canal to the body wall between the setae sacs. 



The alimentary canal comprises a circular mouth opening 

 into a bell-shaped pharynx, followed by a narrow oesophagus, 

 which extends through two segments and leads to a broad 

 stomach with glandular walls, which extends through the sixth 

 segment where it narrows into a straight intestine. As the 

 worms are transparent the movements of the cilia lining the ali- 

 mentary canal may be seen in the living specimen. 



The muscular system is comparatively simple. There is one 

 layer of longitudinal and one layer of circular muscle fibers just 

 beneath the epidermal wall, and single nucleated fibers are con- 

 nected with the setae and hold the various organs in place. 

 These single nucleated fibers are especially numerous in the 

 head and resemble the muscle plates of the Turbellaria. 



The worms were under observation from October until July, 

 and during that time they were constantly reproducing by fis- 

 sion, with sometimes as many as three zooids developing at 

 once. Back of the seventh setigerous segment there is a fis- 

 sion zone in which all the tissues of the epidermal wall are 

 greatly thickened, especially on the ventral side, where they 

 nearly obliterate the body cavity. The new brain arises as a 

 dorsal thickening of the epidermis. No sexual reproduction 

 was observed. 



Methods. — At the slightest irritation the worms will coil in 

 a circle, throw off the contents of the gland cells, and con- 

 tract so violently that the tissues are injured for study. To 



