298 PROCEEDINGS OF THE IHEBICAM ACADEMY. 



three rings, bat oftener of tour. The Bensory ring, which contains a 

 respiratory vesicle, is always th<- broadest ring of the somite. The ring 

 which is usually present, but sometimes wanting, is a narrow one, which 

 has been added at one end of the somite. It is impossible to Bay, without 

 Btudying the nerve distribution, at which end it has been added. Appar- 

 ently, however, it is at the anterior end, for in those animals which I 

 have had an opportunity to examine, the new ring appears to be united 

 more closely with the ring which precedes than with that which follows 

 a sensory ring. Moreover, t lie ring which precedes the sensory ring is 

 usually not so broad as the one which follows it. This is an indica- 

 tion that it is the former rather than the latter which has undergone 

 division.* 



In Trachelobdella (Fig. G, F), according to Blanchard, the somite con- 

 sists sometimes of three, sometimes of six ring-, each of the three prim- 

 itive rings having, in the latter case, divided. The m nsilke remain on 

 the anterior derivative of the original Bensory ring. 



In Cystobranchus (Fig. G, G) the somite consists of seven rings, two 

 of which hear the respiratory vesicles and doubtless have arisen, as in 

 Trachelobdella, from the division of the original sensory ring. The 

 remaining five rings have probably been derived from the two primitive 

 non-sensory rings, — three from one non-sensory ring, two from the 

 other. One of the non-sensory rings must, accordingly, have divided 

 only once, as in Trachelobdella; the other, twice. The position of the 

 genital pores would seem to indicate that the posterior non-sensorv ring 

 is the one which lias divided a second time; but it is impossible to Bay 

 without study of internal structures whether this is really the case or 

 not. 



A typical somite of Piscicola contains, according to Apathy ('88), 

 twelve annuli ; according to Blanchard ( " '. » 1 ) . fourteen. Not havi 

 had an opportunity to examine this genus myself, I do not venture to 

 express an opinion as to the number or morphological value of the rings. 

 Blanchard, however, states that the respiratory vesicle is borne on two 

 rings of the somite, the anterior of which contains the nephropore. This 

 would seem to indicate that the sensorj ring had divided only once, and 

 that the remaining twelve rings (or ten. Apathy) had arisen by repeated 

 division of the two primary non-sensory rings. 



To recapitulate : — We find in the IJhy nchobdellida* that, starting with 

 a -omite of three rings of equal width, increase in the number of annuli 



• v.ul lan t i 'Tim, who was guided by the position of the "septa," also placed the 

 Bomite limits so as to make t«'> rings p ecede the Bensory ring and one follow it. 



