ON PHYMOSOMA VARIANS. 93 



extensile collar, which may be so extended as to entirely cover 

 the head. 



(3) The ectoderm consists of a single layer of cells. This 

 secretes outside a cuticle of varying thickness. The ecto- 

 dermal cells are vaulted, so that spaces are left in which a 

 nutrient fluid might circulate between the circular muscles 

 and the ectoderm. The ectoderm of the lower lip and of the 

 outside of the tentacles is ciliated. 



(4) The skin-glands are of two kinds ; each is formed by 

 the modification of ectoderm-cells, which i-esults in the pushing 

 in of certain of the cells to form a double cup. The inner 

 layer of cells thus produced develops a number of granules, 

 which are extruded through a median aperture. In one kind 

 of skin-gland, those of the introvert, this aperture is sur- 

 rounded by a chitinous ring, which is absent on those of the 

 trunk. 



(5) Rows of hooks set very closely together are found 

 in the introvert ; these are each secreted by a small multi- 

 cellular papilla. 



(6) A skeletal tissue is present in the lip and tentacles. 

 This seems to stiffen these structures, and to form a firm hold 

 for the attachment of the retractor muscles of the introvert. 



(7) The nephridia or brown tubes consist of two parts, the 

 bladder and the secreting part. The former opens both to the 

 exterior and to the body-cavity, the latter opening being 

 shaped like a flattened funnel and ciliated. The secreting 

 part opens only into the bladder. Its walls are lined with a 

 columnar epithelium, the cells composing which are crowded 

 with granules. From time to time a vesicle or bubble crowded 

 with these granules is formed at the free end of the cell, and 

 ultimately breaks off into the lumen of the nephridium, and so 

 passes out of the body. The only other structures found iu 

 the cavity of these organs besides these vesicles, were the 

 ripening generative cells. 



(8) The vascular system consists of a horseshoe-shaped 

 plexus in the lower lip, a similar plexus in the lophophor 

 which gives off branches into each tentacle, and a leservoir 



