56 N. Annandale : The Fauna of Brackish Ponds. [\'f)L. I, 



Mesenteries. — 



The arrangement of the mesenteries in the typical form is, 

 as is frequently the case in the famil}^ subject to many minor 

 irregularities ; but it seems to be a fixed rule in the species that onl}' 

 six pairs of mesenteries are complete, and that they are all, occasion- 

 ally with one or two individual exceptions, fertile. The number of 

 fertile secondar}- mesenteries is variable ; often none of them are 

 fertile, so that Stoliczka was right when he described specimens as 

 having twelve ovaries. The mesenteries of the secondar}- C3'cles in 

 this form are alwaA's smaller than those of the primary cycles, and 

 the retractor muscles of the latter are so feebly developed that as a 

 rule the}" are not visible to the naked e3'e. Mesenterial filaments, 

 more or less perfect in structure, are usually present in those cases 

 in which it is possible to recognize the retractor muscles ; but some 

 of the mesenteries J in all the specimens I have examined, consist 

 merely of the basilar portion, with which they terminate, neither 

 the membranous part between the proximal termination and the 

 retractor, the retractor itself, nor the filament being represented. 

 In the typical form of the species such imperfect mesenteries 

 occur irregularly ; in one specimen a pair was noted which 

 seemed to represent by itself a cycle of which the other mesen- 

 teries were absent. In the new race, on the other hand, it is the 

 rule for all the mesenteries except the six primary pairs to be in 

 this rudimentary, or possibly vestigial condition. Only exception- 

 ally do any of the secondary mesenteries bear retractor muscles, 

 filaments or gonads. This condition of affairs considerabl}" in- 

 creases the lumen of the coelenteron, which is further enlarged by 

 another peculiarity nameh^ the thinness of the mesoderm in the 

 mesenteries. In the typical form of the species, this layer rather 

 increases in transverse diameter as it juts out into the mesen- 

 teries, and maintains a porportionately considerable breadth the 

 whole way between the basilar and retractor muscles. In the new 

 race, however, although it bulges out and takes on a dendritic 

 form in the region in which it supports the basilar muscles, it de- 

 creases greatl}^ in thickness between the distal extremity of the latter 

 and the base of the retractors. Indeed, to such an extent is this the 

 case that in what may be called the membranous part of the 

 mesentery, the mesoderm appears in transverse sections as an ex- 

 tremely delicate filament. There are, of course, differences in the 

 transverse diameter of this layer, so far as the mesenteries are 

 concerned, in different regions of the column ; but the differences 

 just described are very much more conspicuous than any of a local 

 nature. 



Both internal and external mesenterial stomata are present in 

 both forms. 



The structure of the mesenterial filaments calls for no special 

 remark either as regards the species as a whole or as regards the 

 two forms thereof. It agrees closely with that which has been des- 

 cribed by O. and R. Hertwig (3), and subsequently by others, in the 

 cases of different members of the Sagartiidse. The onh' points in 



