732 MR. J. p. MCMURRICH ON THE SYSTEMATIC 



other two such pairs. These two individuals have, then, six 

 pairs of perfect mesenteries and a number of imperfect, ones 

 disposed unsymmetrically. It is to be noted that this arrange- 

 ment is that found in sections passing through the aboral half 

 of the stomodfeum ; higher up, close to the bases of the tentacles, 

 a number of additional rudimentary mesenteries may be found, 

 but these need not be considered at present. 



In the third individual (A) there were but five pairs of perfect 

 mesenteries in sections passing through the aboral half of the 

 stomodfeum. Higher up, however, six occurred, with repre- 

 sentatives of two cycles of imperfect ones in the intervals on 

 either side of the fully-developed directives and in that between 

 YI. and V., but only a single feeble pair in each of the other 

 intervals. Following the series of sections aborally it is found 

 that the change from the conditions showing six pairs of perfect 

 mesenteries to that showing only five is due to the disappearance 

 of a member of each of the pairs II. and III. together with the 

 intervening impei-fect mesenteries, the remaining members of 

 the perfect pairs thus being brought into apposition and seeming 

 to form a, single pair. 



The explanation which seems to fit these cases is that each is 

 an example of a,n individual that has undergone longitudinal 

 fission. The half of the body with the deep siphonoglyph and the 

 I'epresentatives of three cycles of mesenteries shows the normal 

 symmetrical arrangement characteristic of the species, and the 

 ii-regularities and feeble development in the other half are 

 indications of an attempt to regenerate the disturbed symmetry. 

 It may be concluded that in the original individuals developed 

 from ova there were three cycles of mesenteries arranged 

 hexamerously, only those of the first cycle being perfect. The 

 reproductive elements were not sufiiciently developed in any of 

 the individuals collected at Woods Hole to allow of a determina- 

 tion of their distribution, but in specimens collected at Plymouth 

 it was found that they were limited to the pairs of the second 

 and third cycles, i.e., to the imperfect mesenteries. It may be 

 added that both oral and marginal stomata occurred in the 

 perfect mesenteries. Acontia are present, but are not abundant. 



The structural features, then, that may be supposed to be 

 characteristic of individuals that have not undergone fission are 

 (1) the occui-rence of a well-defined fosse; (2) the absence of a 

 mesogloeal sphincter; (3) the occurrence of but six pairs of 

 perfect mesenteries, which are also sterile, the reproductive 

 elements being limited to the imperfect cycles. But before 

 considering the significance of these features it will be well to 

 consider the geographical disti'ibution of the species. 



Yerrill's original description of Sagartia lucice (1898) was 

 based on individuals collected in Long Island Sound. He states 

 that his attention was first directed to it in 1892 and expresses 

 his conviction that it did not occur in any quantity in that 

 region throughout the period 1865-1890. Furthermore it was 



