THE STRUCTURE AND AFFINITIES OF SACOOCIERUS. 417 



Marion and Bobretzky correctly described the head cavity, 

 and believed its function to be erectile ; contraction of the 

 ampullje driving the contained fluid into the tentacular 

 canals, and so straightening out the tentacles. This inter- 

 pretation is very probably correct, but I have never myself 

 observed any very manifest signs of such action. Langerhans 

 added the observation that the wall of the head cavity is 

 provided with an outer cuticular lining. Staining with 

 nigrosin shows the covering coloured dark blue like the 

 cuticle. Fraipont has figured transverse sections of the 

 system, the lumen being drawn as if filled with a reticulum. 



Whilst fully confirming most of these authors' statements, 

 it may be added that the contents seemed to me to be not 

 colourless, as described, but pale pink ; and further, that the 

 " fluid" is really composed chiefly of large cells with granular 

 and very fluid contents closely packed together (figs. 1 and 

 17). No doubt there is some intervening liquid, but it does 

 not appear in sections. 



The morphological significance of this head cavit}^ is not 

 easy to determine. It seems to me probable that it represents 

 the specialised ccelomic cavity of the peristomial segment 

 (see p. 404). 



Excretory and Genital Organs. — Our knowledge of 

 these organs is due entirely to the observations of Marion 

 and Bobretzky, some of which Langerhans confirmed. This 

 description, on the whole, is remarkably good, but it is 

 incomplete and incorrect in a few points. 



These authors found that the sexes are separate ; that the 

 testes and ovaries are developed from the ccelomic epithelium 

 on the posterior surface of the septa; that in the male there 

 is, in the region behind the oesophagus, on either side of each 

 segment a protrusible penis with sheath, leading into a 

 ciliated duct which enlarges to form a vesicula seminalis, 

 continued into an open funnel, " dispose d'apres la forme 

 typique des organes segmentaires " (10). In the oesophageal 

 region they found ordinary nephridia opening at the same 

 level as the penis. Naturally they believed that the male 



