igoy.] Records of the I ndiayi Museum. 5I 



ably elongated epithelial cells provided with cilia, which are parti- 

 cularly long and active towards the upper limits of the column. In 

 the typical form, the number of gland cells interspersed in the epi- 

 thelium is perhaps greater than in the variety. In the former, the 

 cells lining the intermesenterial spaces are markedly longer, and 

 contain more zooxanthella^, than those lining the intramesenterial 

 spaces. This difference is not so clear in the variety but appears to 

 exist to a slight extent. In both forms the zooxanthellce do not 

 encroach upon the basal part of the cells. 



An important point to be noted is that the differences in struc- 

 ture of the column are much more marked in the case of full-grown 

 individuals of the two forms than they are in that of very young in- 

 dividuals of the variety and adults of the typical form. As can be 

 seen from the figures on plate iv, young individuals of the variety 

 measuring about 10 mm. in length when contracted, are only about 

 four times as long as broad, their proportions being, however some- 

 what variable. In full-grown specimens of the same form, however, 

 the length is at least ten times the transverse diameter. When 

 strongly contracted the column of the young individuals assumes a 

 barrel-shaped outline wliich does not differ very greatly from the 

 conical outline of the typical form in same state, and the younger the 

 individual is, so far as my experience goes, the less is the lerigth in 

 excess of the transverse diameter. It is only well-grown individuals, 

 of over 4 cm. in length when they are contracted, which can be called 

 wormlike, and as will be shown later, contraction takes a dift'erent 

 course in full-grown examples of the variety than that which occurs 

 in young examples of the variety or full-grown individuals of the 

 original form. In the typical form and in the young of the variety, 

 the column is able to stand vertically upright, but in larger indi- 

 viduals of the variety this is impossible without artificial support. 



Muscles. — 



The circular muscular layer of the mesoderm of the column lies 

 within the nervous layer of the same structure and, in the variety, 

 occupies the greater part of the mesogloea. In the typical form it 

 is relatively less extensive. In the typical form, moreover, the 

 muscle fibres appear to form a continuous sheet, but I am not 

 quite confident as regards this point. In the variety, however, it 

 is easy to see in living and even in well-preserved specimens that 

 this muscle consists of a large number of parallel strands lying 

 closely adjacent to one another in a vertical series. I am not 

 referring to the sphincter, which is formed by a folding of the 

 muscle accompanied by a parallel folding of the whole mesoderm, 

 but to the circular muscle of the column below the sphincter. 



In the typical form of the species the sphincter is not visible 

 externally and its folds are so shallow and commence so gradually 

 below, that it is difficult to say at what point it begins to become 

 differentiated. This is also the case as regards young specnnens 

 of the variety less than five millimetres long : but even m these it 

 is more powerfully developed. In full-grown specimens of the 



