682 POLYZOA. 



the continuations of the two limbs are formed by perforated 

 mesoblast cells. 



According to Fol these organs originate in aquatic Pulmonata 

 as a pair of invaginations of the epiblast, slightly behind the 

 mouth. Each invagination grows in a dorsal direction, and after 

 a time suddenly bends on itself, and grows ventrahvards and 

 forwards. It thus acquires its V-shaped form. 



In the terrestrial Pulmonata the provisional excretory organs 

 are, according to Fol, formed as epiblastic invaginations, in the 

 same way as those in the aquatic Pulmonata, but have the form 

 of simple non-ciliated sacks, without internal openings. 



The permanent renal organ of the Mollusca consists typically 

 of a pair of tubes, although in the majority of the Gasteropoda 

 one of the two tubes is not developed. It is placed considerably 

 behind the provisional renal organ. 



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. II. No. 268) tend 

 to shew that it is developed from a mass of mesoblast 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 the 

 organ arises as a solid mass of epiblast cells. Lankester 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 Cephalopoda 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 



