HEEATEEOBRANOHUS SHRUBSOLIT. 179 



The cheetse all arise from sacs, each group from one sac, 

 firmly implanted in the body-wall and projecting into the body- 

 cavity, though the ventral sacs do not project so far as the 

 dorsal. The wall of each sac is supplied by muscles, by means 

 of which the sac can be moved in and out as a whole. Springing 

 separately from the body-wall behind both dorsal and ventral 

 chsetse-bundles slightly dorsad, of the dorsal ones and ventrad 

 of the ventral ones, are the membranous lobes known either as 

 " cirri " or '' parapodial laminae. ^^^ They can readily be seen 

 in the first few segments, but then gradually grow smaller, 

 and are not found in the posterior region of the body. It is 

 difficult to determine exactly in which segment they cease to 

 exist, and whether this is constant in all, since, to see them 

 clearly, the animal must be living and moving, and it is then 

 not easy to count them. When the animal is killed the lobes 

 become, by the position taken up by the worm, very difficult 

 to see and be certain of. The dorsal ones are much closer to 

 one another, i. e. nearer to the median line, anteriorly than 

 posteriorly, and in the 2nd segment (the first one in which they 

 exist) they are so close together that they seem to form, or form 

 part of, a collar, which is therefore dorsal, and quite distinct 

 from the ventral collar of the 1st segment (figs. 1 and 12, d. coll.). 

 There are very minute stiff hairs on all these lobes, resembling 

 cilia, but without their movement. Such hairs are also found 

 elsewhere on the cuticle of the body-wall. 



Internal Anatomy. — The body-wall consists of — (1) An 

 outer epidermic layer of cells, in parts more than one layer 

 thick, with a fine cuticle (figs. 5, 6, 7, and 8, epid.). The 

 epidermis is thicker on the ventral surface than elsewhere. 



parapodium of an Errant annelid, e.g. Nereis or Phyllodoce ; and it is difficult 

 to say whether this homology exists without first deciding, by the comparison 

 of a large number of forms, to which families of the Errantia the Spionidse are 

 most nearly allied, taking the Spionidse to be, as they probably are, the living 

 representatives (though probably degenerate) of the most primitive of the 

 Sendentaria. A cirrus may vary so much both in form and position that we 

 can see no reason why these membranous lobes in the Spionidse should not 

 represent cirri ; but this, of course, does not alone in the least prove them 

 to be true cirri. 



