DORYMENIA ACUTA. 103 



zone immediately behind the retractors. On the other hand, after passing 

 backward, the various groups of fibres become attached to the bodj' wall or to 

 the cloacal wall in the neighborhood of the opening of the spicule sheath. 

 Numerous other strands occur in this region whose function it is to widen the 

 cloaca, enabling the spicule to be exposed while others bring about a counter 

 movement. Those active in the first operation consist of many bands passing 

 radially from the wall of the cloaca to become inserted in the body wall, and 

 others which pass from the cloacal wall anterioi-ly to blend with the somatic 

 muscles. The remainder, responsible for the reduction of the cloacal cavit.y, 

 comprise many fibres which attach to the walls of the cloacal cavity, and 

 passing backward unite with the body wall on each side of the forward border 

 of the cloacal opening. 



A pair of curious vesicles, irregular in form but of comparatively large size, 

 occur one on each side of the body wedged in between the cloacal wall, spicule 

 sheath, and coelomoduct and, as sections show (Plate 15, fig. 4), they are sepa- 

 rated from each other by a thin vertical wall. On the posterior face of each a 

 short slender tube communicates with the cloaca (Plate 9, fig. 2). In immature 

 individuals or those not sexually active the walls are comparatively thin, and are 

 composed of cells cubical or low columnar in form without any distinct signs 

 of glandular activity; but as the breeding season approaches the walls become 

 much thickened and each cell develops some substance which gives it a longi- 

 tudinally striated appearance. This material remains unstained in haematoxy- 

 lin and as it forms crowds the nucleus to the distal end. There is some evidence, 

 though scanty, that this secretion is poured into the diverticulum and there 

 becomes transformed into a darkly staining mucus-like substance which everj-- 

 where lines the walls. Here and there are blood sinuses filled with corpuscles 

 especially in the region of the opening into the cloaca where the cells are lower, 

 without secretory products and covered with an abundance of cilia. 



The fact that these modifications occur simultaneously with those of the 

 gonoducts strongly suggests that these organs in some way play a part in the 

 reproductive process. They may function as uteri but obviously such con- 

 jectures are of very little value at the present time. 



There is some reason to believe that the type of coelomoduct found in 

 Chaetoderma is more like that of Chiton, and accordingly of a more primitive 

 type than in the Neomeniina where they are provided with seminal receptacles, 

 glands often of enormous development and spicula, in some cases, provided also 

 with glandular appendages. It is interesting to note that in an immature 



