i5 





arrangement of the quartette may be represented by the formula B — m — b — M. The same 



formula answers also for the succeeding quartettes, but it will be observed that in each quartette 



there is a gradual diminution in the length of each of the four kinds of mesenteries compared 



with the corresponding mesentery of the preceeding quartette. In other 



words the mesenteries of each kind diminish gradually in length towards 



the dorsal mid-line, the length of any one kind being proportional to 



that of the preceeding quartette. Hence, as regards their lengths four 



different kinds of deuterocnemes may be recognized. Along with this 



arrangement is to be noted the fact that none of the deuterocnemes 



approach the aboral pole. 



In C. verrillii the arrangement of the mesenteries in the 

 quartettes is essentially the same as in C. membranaceus, being expressed 

 by the formula B — m — b — M, and the same correlation in length of 

 the individual mesenteries of each set also occurs. A differance is 

 found, however, in the fact that three of the longer macrocnemes on 

 each side extend almost to the aboral pole and there is a gradual 

 diminution of the remainder of this set towards the dorsal mid-line 

 (See Kingsley, 1904). 



In P . fimbriatus (See Text-figure II, p. 1 2) the arrangement 

 is similar to that of C. membranaceus, although in the single example 

 carefully examined the regular diminution in length of the various 

 kinds of mesenteries was interrupted in the third, fourth, fifth and 

 sixth quartettes, but resumed in the succeeding ones. None of the 

 deuterocnemes approach the aboral pole. 



In P. monostichus, so far as I was able to ascertain the arrangement, it was similar to 

 that of C. verrillii, but an account of the details of the arrangement both in this form and in 

 the preceeding one will be given later with the description of the species (See Text-figure X). 



In these four species, then, the arrangement of the quartettes was according to the 

 formula B — m — b — M. In the two species now to be mentioned we find a different formula is 

 necessary, namely, B — M- — b — m ; that is to say, the longer macrocneme precedes the shorter 

 one in each quartette. This is the case in C. lloydii (Van Beneden, 1898), and in this form 

 we find that while the two sets of brachyenemes have an independently progressing length- 

 diminution towards the dorsal surface, the two sets of macrocnemes form one diminishing series, 

 the shorter macrocneme of the first quartette being longer than the longer one of the second 

 quartette and so on. Furthermore, the macrocnemes of the first two quartettes extend to the 

 neighbourhood of the aboral pole, but with the third quartette a sudden diminution in length 

 comes in, none of the mesenteries from this one extending beyond the middle of the column. 



In C. americaims (Mc Murrich, 1890; see also Text-figure IV) the quartette arrangement 

 is also B — M — b — ;//. The brachyenemes are all very short, extending but a short distance 

 below the lower border of the stomatodaeum, so that the amount of difference in length of 

 those of each quartette is relatively small though appreciable, and, as a rule, the longer one 



15 



Fig. III. 



Diagram showing the arrangement 



of the mesenteries of 



Cerianthus membranaceus. 



Adapted from Faurot. 



