1882.] Structure, Develojjment, and Affinities of PhoronU. 377 



Dr. Hatschek believed that the whole organ was formed from the 

 mesoblast cells mentioned above. 



Vascular System. 



The blood-vessels are all formed from the spianchnopleure. 



The blood-corpuscles found in the vessels immediately after meta- 

 morphosis arise from mesoblast cells in front of the septum. 



They form in masses which vary in number and position with the 

 species, and lie free in the body-cavity held together by processes 

 resulting from incomplete division. Each corpuscle has a nucleus, 

 and with the growth of the larva the haemoglobin colour gradually 

 develops. 



The vessels arise as splits in the spianchnopleure. The adult condition 

 is reached partly by constrictions, partly by outgrowths from these. 

 Thus we have at the close of larval life the blood-system in the 

 following condition: — 



1. Blood-corpuscles aggregated in two or more masses, lying free 

 in the body-cavity of the praaoral lobe, i.e., in front of the septum. 



2. A blood-vessel formed on the dorsal wall of the stomach, a 

 marked structure in the larva. 



3. The splanchnopleure sac, which in the region of the stomach 

 forms a loose sac surrounding the gut. 



4. Cascal prolongations of this sac. 



5. Caecal prolongations into the rudiments of the adult tentacles. 



Lophophore. 



The larval tentacles are produced in pairs always towards the dorsal 

 line, so that the most dorsal are the youngest. This is also the case 

 in the adult Phoronis. 



But the first rudiments of the adult set appear laterally, and new 

 pairs are added both ventrally and dor sally to this pair, so that the 

 oldest adult tentacles are not the most ventrally situated. 



Full-formed Larva and Metamorphosis. 



Finally we get the full grown free swimming larva, whose chief 

 organs and their relative positions I shall briefly recapitulate. 



The mouth and anus are at opposite ends of the ciliated body. The 

 mouth is overhung by a large praeoral lobe, whose margin is slightly 

 thickened, and bears longer cilia than on the rest of the surface. This 

 margin corresponds to the velum (praeoral ring) of other larva. 



I reserve the discussion of Kleinenberg's paper* on the origin of the 



* N. Kleinenberg, " Sull' Origine del Sistema Nevvoso Centrale degli Annelidi." 

 Eeale Acad. d. Lincei, 1881. 



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