50 N. Annandale: The Fauna of Brackish Potids. [Vol. I, 



the nematocysts (o/). ci7., plate xi). The secretion of the gland 

 cells mixed with the threads of the nematocysts forms a cover- 

 ing for the column, which, however, is only temporary, and has 

 not the characters of the so-called cuticle found in some Actinians. 

 Unless specimens are very carefully preserved, the whole of the nem- 

 atocysts of the column and tentacles are forced out of the ectoderm 

 without rupturing, and appear in transverse section to form a sep- 

 arate layer, bound together by slime secreted by the gland cells but 

 external to and distinct from the ectoderm. If living specimens are 

 examined, it will be found that there is no such layer under natural 

 conditions, but that the nematoc^'sts are interspersed with the epi- 

 thelial and glandular cells. The temporary protective covering is 

 not formed of the nematocysts, but only of their threads and of 

 slime, often with foreign bodies enclosed. In the isolated race the 

 ectoderm consists of a layer of cells parallel to the mesoderm. 

 In the typical form, however, this layer is thrown, all round the 

 periphery of the column, into a series of transverse folds, the func- 

 tion of which I will discuss later. The number of nematocysts and 

 also of gland cells present in this region is perhaps greater in the 

 typical form than in the variety. The suckers, which are as a rule 

 absent in young individuals, consist, in both forms, of relatively 

 deep folds of the ectoderm separated by a space from the meso- 

 derm ; they are oval in outline, their main axis being at right 

 angles to that of the column. It is very difficult to detect the cin- 

 clides in preserved material, but in life they are easily distinguished 

 as transversely elongated sHts with tumid lips. In structure they 

 closely resemble the suckers except that they are perforate ; the 

 mesogloea beneath them is much vacuolated. The vertical rows 

 of suckers, at any rate in the variety, usually correspond to the 

 inter-, those of the cinclides to the intramesenterial spaces ; but 

 I have been unable to convince myself that this arrangement is 

 absolutely constant. In the typical form of the species cincUdes 

 and suckers occur on all parts of the column, the former being 

 particularly numerous near the two disks ; but in the new race 

 both structures are confined to the upper half of the column below 

 the region of the sphincter. 



The thickness of the mesoderm is not more than moderate in the 

 typical form ; in the variety it is rather less, but the mesogloea swells 

 out somewhat irregularly in many of the inter- and intramesen- 

 terial spaces in such a way that the whole of the layer in such spaces 

 has a roughly spindle-shaped outline in transverse section. In both 

 forms the nerve cells situated towards the external limits of the 

 mesoderm are large and numerous, and in both the mesogloea itself 

 has a distinctly reticulo-fibrillar structure and contains, especially 

 externall}^, a number of irregularly placed vertical spaces and 

 channels. In the typical form of the species, the wider folds of 

 the ectoderm rest on slight projections and concavities in the 

 mesoderm, while in both forms broad mesodermal " bays " occur 

 on the endodermal surface. 



The endoderm of the column in both forms consists of consider- 



