278 W. B, BENHAM. 



tive than the remainder of the earthworms, and closely 

 resembles Stylaria in the condition of its prostate. 



As I said above, we have practically no embryological data 

 on which to found our theories as to " primitive " and 

 '' secondary '' characters in the earthworms. But there is 

 one organ on which we have definite information, and that is 

 that the dorsal blood-vessel is in Criodrilus formed by the 

 fusion of a double vessel. Now, in several earthworms we 

 find this double condition of the vessel. 



In Acanthodrilus multiporus and in Deinodrilus 

 benhami there is a pair of dorsal vessels; in A. dis- 

 similis this vessel is doubled in every somite, fusing at the 

 septa : this condition is also present in the anterior somites 

 of Microchseta rappi, and according to Beddard in Peri- 

 chseta coerulea (Pleurochseta moseleyi), and this seems 

 to have been the chief reason, in addition to its plectouephric 

 condition, for regarding Acanthodrilus as the more primi- 

 tive genus. 



In which worm are any of these organs retained in their 

 most primitive condition ? I think that Moniligaster sup- 

 plies the answer in most points. The setie, clitellum, sperm- 

 ducts, and prostate are all in agreement with the above- formu- 

 lated conditions. The gizzard, too, is very diff'erent from what 

 we find in other worms ; its walls appear to be much less 

 muscular than is usually the case ; it is less marked, extends 

 through several somites, and recalls the enlarged intestine of 

 water-worms, with its wall only slightly thicker than the pre- 

 ceding oesophagus.^ 



Thus, on the whole, I am inclined to regard Moniligaster 

 as the most primitive living earthworm, or rather as 

 approaching most nearly to their original ancestor. At the 



* I should add that the anterior gizzard mentioned by Perrier has not been 

 found in any of the species recently described — seven by Bourne, one by 

 Horst, one by Beddard ; and it is probable that he mistook for gizzard a 

 mere dilatation of oesophagus, as was the case in his description of Perionyx. 

 Here he stated that the gizzard was in Somite xii ; llosa found here a 

 swelling only, the true gizzard being in vii. 



