656 GERARDA STIASNY-WIJNHOFF 



as the most advanced ones ; I, however, believe that the 

 formation of a dorsal blood-vessel takes place in the Pelagica ; 

 that it has been formed in two ways, either in relation to the 

 rhynchocoelomic cavity, or quite separately. This last way 

 is represented in three aberrant genera, Armaueria in the 

 Pelagica, Malacobdella, the Bdellonemertean, and Siboga- 

 nemertes, the representative of a new group of Reptantia. 

 Perhaps other genera passed this stage to acquire a rhyncho 

 coelomic vessel afterwards, as might be plausible in Prostoma. 

 Some genera, however, seem to have got this rhynchocoelomic 

 vessel directly, as is shown in Pelagonemertes rollestoni 

 and the nearly related genus Natonemertes with a short, 

 blind-ending proboscidian blood-vessel, or in the family of the 

 Balaenanemertidae, where a dorsal vessel is absent in 

 B. chuni, and in other species of the same genus a blind 

 rhynchocoelomic vessel is present as well as in Probalaena- 

 nemertes and Parabalaenanemertes. Another fact of interest 

 in the blood-vascular system, and which seems to demonstrate 

 how the organisms of this group try to obtain a certain result in 

 different ways, is the development of what B r i n k m a n n calls 

 ' Ovarialschlingen '. He demonstrates in Dinonemertes 

 investigator is that the lateral blood-vessels in the 

 gonadial region make large, irregular loops between the 

 ovaries and the entodermal sacs. These loops that convey 

 the nutrition from the sacs to the ovaries are absent in all other 

 forms ; but I found them also in Siboganemertes. It is supposed 

 that the vascular loops between the dorsal and lateral vessels 

 of other Nemerteans have the same purpose, and Brink- 

 m a n n remarks that this fact states D o 1 1 o " s law of irrever- 

 sibility, as the regular loops that once disappeared did not 

 return, but another structure took on the same function. 

 What we find in Dinonemertes and Siboganemertes can 

 perhaps just as well be the beginning of what results in vascular 

 loops between the vessels. So everywhere I reach the same 

 result ; the Pelagica show the development of every organ, from 

 the primitive stages known in Palaeonemerteans to the 

 specialized features of Drepanophoridae and Monostilifera : the 



