PHORONIDEA. 545 



lying between the right and left lateral mesenteries and containing 

 the rectum, and a right and left chamber between the oesophageal 

 and stomach part of the median mesentery and the right and left 

 lateral mesenteries. As the lateral mesenteries end before the aboral 

 apex of the body is reached, all three chambers are in communication. 

 In addition to the mesenteries, bridles of connective tissue pass 

 across the body-cavity from the body- to the gut-wall. The coelom 

 contains a corpusculated fluid, in which the reproductive cells are 

 often found floating. 



The vascular system consists of two main longitudinal trunks, 

 one lying in the rectal chamber between the two limbs of the 

 alimentary canal (a/), and the other on the left side of the oeso- 

 phagus (e/). These vessels are contractile, and are continuous 

 aborally; they both give branches to the gut- wall and gonads. 

 Anteriorly they pass through the septum, and are both connected 

 with the single vessel found in each tentacle. The blood consists 

 of a colourless plasma, containing nucleated red (haemoglobin) 

 corpuscles in suspension. 



The renal organs consist of two nephridia, which also act as 

 generative ducts and open externally on each side of the anus. 

 Each nephridium, after passing aboralwards through the septum, 

 opens into the body-cavity by two openings. One of these the 

 smaller (ne.f} is into the lateral chamber of the body-cavity, the 

 other and larger (ne.f) into the rectal chamber. 



Phoronis is hermaphrodite, and the gonads lie on the left side of 

 the stomach in the aboral region of the body-cavity the ovaries (0) 

 on one side of the oesophageal blood vessel, and the testes (T) on 

 the other. They consist of special developments of the coelomic 

 epithelium overlying certain caecal branches of the oesophageal 

 vessel. The reproductive cells are dehisced into the coelom, and 

 pass out by the nephridia. 



Development. The eggs are ^mall, and are probably fertilized 

 externally. They undergo the first part of their development 

 entangled in the arms of the mother. They are then hatched out 

 as free-swimming larvae, which after a certain period of free life 

 yndergo a remarkable metamorphosis, and acquire the adult form 

 and habit. The larva is called Actinotroclia. 



There is an invaginate gastrula, and the blastopore, which assumes 

 a slit-like form, closes up behind, but remains open in front as the 

 mouth. The anus is formed as a pit at the hind end of the closed- 

 up portion. The mesoderni arises partly as cells budded off from 



2 N 



