] 



If est}' is. 225 



septal or anterior pair, and one preseptal or posterior pair, each 

 secondary vessel connecting with the ventral vessel. The perioas- 

 tric vessel in somite VI connects directly the ventral and do^rsal 

 vessels (Fig. 26), while in the anterior somites the perigastric vessel 

 connects the forks of the ventral vessel. Thus somite VI differs 

 from all other somites, which contain the two primary vessels, in 

 that there is only one pair of perigastric vessels, all posterior 

 somites having two pairs. A similar arrangement is seen in Rhyn- 

 cJiebnis limosella;'^ where the six and seven somites have only one 

 pair of perigastric vessels, but in Sutroa rostrata only the 6 ante- 

 rior somites are characterized by the existence of a single pair of ' 

 perigastric vessels, t . The perigastric vessels in the posterior somites 

 in Sutroa alpestris connect with the ventral vessel. The dorsal parts 

 of these perigastric vessels are feathered or ramified, and the ventral 

 part of the postseptal or anterior pair is similarly but somewhat less 

 extensively ramified (Figs. 24, 27, 28). Of the posterior pair the 

 ventral part is simple, not ramified (Figs. 27 and 28.) The peri- 

 gastric vessel in somite VI is not ramified. 



Thus, as regards the perigastric vessels, Sutroa differs from Rhyn- 

 chelmis m having both the postseptal and preseptal pairs ramified, 

 Rhynchelmis having only the preseptal pair ramified. In Rhyn- 

 chelmis limosella only the anterior perigastric pair connects with the 

 ventral vessel. In Sutroa rostrata neither pair connects with the 

 ventral vessel, while in Sutroa alpestris both pairs connect with it. 

 It \yill thus be seen that generic characteristics cannot always be 

 derived from the branching or from the connections of the vessels. 

 As specific characteristics, however, they are of the greatest impor- 



tance. 



The branches of the posterior pair of perigastric vessels are less 

 numerous but rather more prominent than those of the anterior pair, 

 the latter being by far the strongest vessels. There are in e\-er)' 

 somite, beginning with XII,' two or three pairs of gastric vessels 

 attached to the intestine. There are also tertiary perigastric vessels 

 along the reproductive and conductive organs, but none surround- 

 ing the nephridia or segmental organs. 



Recapitulating, it may be said that in Sutroa alpestris the vascu- 



Vejdovsky, Anatom. Stud. loc. cit. page 344 

 tEisen On Sutroa, loc. cit. fig. 4. 



