ANNELIDA 



309 



fully formed animal, we have already identified as the apical 

 plate. From it two cords extend backwards, embrace the 

 mouth-opening, and unite with the ventral cord. In this 

 way are formed the oesophageal connectives, which are un- 

 usually large in the Echiuridm (Figs. 145 and 146 sc). 



The anal vesicles, which open to the 

 exterior along with the intestine (Figs. 

 145 and 146), do not arise, as was sup- 

 posed, from the intestine, but are 

 formed in the somatic layer of the 

 mesoderm. They lie here in the ter- 

 minal segment of the body as two 

 compact cylindrical structures, which 

 later become hollow and unite with 

 the ectoderm on either side of the anus. 

 At the same time they grow inward. 

 Their middle part is distended, atid the 

 inner end opens free into the body 

 cavity by means of a ciliated funnel 

 (Fig. 146 ab). Their entire mode of 

 origin proclaims the anal vesicles to 

 be nephridia, which only secondarily 

 entered into connection with the hind- 

 gut. 



The intestine is no longer so wide in 

 comparison with the entire body, as is 

 to be seen in Fig. 146 ; on the con- 

 trary, it has grown more in length and 

 makes several turns, which subse- 

 quently are still naore emphasized. In 

 the meantime the larva has also altered 

 externally, in that its transverse dia- 

 meter has decreased in proportion 

 diameter (Figs. 145 and 146). On the surface the rows of 

 dermal papillfe become noticeable, and, just as in the 

 Chaetopoda, the uncinate setge which are formed in the 

 setigerons sacs (immediately under the ectoderm) break 

 through to the outside (Fig. 146). The further development 

 consists first of all in an active growth of the hinder portion 



Fig. U6.— Larva of 

 Ec]üu\-us (after Hats-- 

 chek). a, anus ; ah, anal 

 vesicle ; b, circle of setse 

 at the hind end of the 

 body ; d, intestine ; m^ 

 mouth; 11, ventral chain 

 of ganglia ; sc, oesopha- 

 geal connective ; sp, api- 

 cal plate ; vh, ventral or 

 uncinate setfe. 



to its longitudinal 



