86 MARGARET 0. COLLCUTT. 



The chitin which is situated beneath such a degenerating 

 mass becomes somewhat cup-shaped by unequal growth (PI. 1, 

 fig. 2). Gradually the edges of the cup grow over, only leaving 

 the mass in the cup attached by a narrow bridge to the upper 

 coenosarc, until finally the edges of the cup meet and coalesce, 

 and so isolate the degenerate mass of coenosarc. These con- 

 stricted masses consist only of the deeper parts of the coeno- 

 sarc, i. e. the lower ectodermal layer and the endoderm, and as 

 they degenerate the boundaries between the cells become 

 obliterated, the protoplasm becoming very granular, and the 

 nuclei losing their distinctive appearance. 



The Edge of the Colony. — At the edge of the colony 

 where growth occurs the skeleton is very thin, though thick- 

 ened at frequent intervals into short, solid spinules ; here, too, 

 the coenosarc is generally thinner, and the polyps are few and 

 small. 



Extending over the surface of the coenosarc of this region is 

 a thin membranous layer of chitin, which is connected at 

 many points with the tips of the spinules of the skeleton. 



In this region the skeleton is occasionally raised up into 

 conical smooth spines, which are fenestrated at their bases, so 

 that portions of the basal coenosarc, consisting of lower ecto- 

 derm and of endoderm, can penetrate into the interior of the 

 spines (PI. 1, fig. 3). 



These spines are partially covered by coenosarc, which, how- 

 ever, does not extend over their tips; a thin layer of chitin 

 spreads over the surface of the coenosarc, but is not continued 

 over the naked tips of the spines (PL 1, fig. 3). 



Strethill-Wright (10) mentions hollow chitinous spines filled 

 with coenosarc, but he describes them as being derived only 

 from the upper layer of ectoderm. 



The colony is derived at its growing edge from a number of 

 coenosarcal tubes clothed with a chitinous perisarc; these 

 tubes branch about irregularly over a large portion of the 

 whelk-shell, into the cavity of which they extend ; ectoderm 

 covers their tips, and they appear to secrete their perisarc as 

 they grow. 



