EXCRETORY ORGANS. 563 



Each tube, in its most typical form, opens by a ciliated funnel into ] 

 the pericardial cavity, and has its external opening at the side of the [ 

 foot. The pericardial funnel leads into a glandular section of the' 

 organ, the lining cells of which are filled with concretions. This \ 

 section is followed by a ciliated section, from which a narrow duct 

 leads to the exterior. 



As to the development of this organ the same divergence of 

 opinion exists as in the case of the provisional renal organ. 



Rabl's careful observations on Planorbis (Vol. i. No. 268) tend to 

 shew that it is developed from a mass of raesoblast cells, near the end 

 of the intestine. The mass becomes hollow, and, attaching itself to 

 the epiblast on the left side of the anus, acquires an opening to the 

 exterior. Its internal opening is not established till after the 

 formation of the heart. Fol gives an equally precise account, but 

 states that the first rudiment of tlie organ arises as a solid mass of 

 epiblast cells. Lmkester finds that this organ is developed as a 

 paired invagination of the epiblast in Pisidium, and Bobretzky also 

 derives it from the epiblast in marine Prosobranchiata. In Cephalo- 

 poda on the other hand Bobretzky's observations (I conclude this 

 from his figures) indicate that the excretory sacks of the renal 

 organs are derived from the mesoblast. 



Polyzoa. Simple excretory organs, consisting of a pair of ciliated 

 canals, opening between the mouth and the anus, have been found 

 by Hatschek and Joliet in the Entoproctous Polyzoa, and are de- 

 veloped, according to Hatschek, by whom they were first found in 

 the larva, from the mesoblast. 



Brachiopoda. One or rarely two (Rhynchonella) pairs of canals, 

 with both peritoneal and external openings, are found in the Brachio- 

 poda. They undoubtedly serve as genital ducts, but from their struc- 

 ture are clearly of the same nature as the excretory organs of the 

 Chsetopoda described below. Their development has not been worked 

 out. 



Chsetopoda. Two forms of excretory organ have been met with 

 in the Cha^topoda. The one form is universally or nearly universally 

 present in the adult, aud typically consists of a pair of coiled tubes 

 repeated in every segment. Each tube has an internal opening, I 

 placed as a rule in the segment in front of that in which the greater* 

 part of the organ and the external opening are situated. 



There are great variations in the structure of these organs, which 

 cannot be dealt with here. It may be noted however that the 

 internal opening may be absent, and that there may be several in- 

 ternal openings for each organ (Polynoe). In the Capitellidse more- 

 over several pairs of excretory tubes have been shewn by Eisig (No. 

 512) to be present in each of the posterior segments. 



The second form of excretory organ has as yet only been found in 

 the larva of Polygordius, and will be more conveniently dealt with in 

 connection with the development of the excretory system of this 

 form. 



86—2 



