8 PHORONIDEA. 



There are a few points connected with the rise of the organs which must 

 be described a little more in detail. The nephridia of Phoronis which have 

 recently been carefully studied by Caldwell (No. 1) and Cori (Nos. 4 and 4a, 

 and Ectoproc. Bryoz. Lit., No. 46) are, in the adult, paired, looped, ciliated 

 canals opening externally near the anal aperture (Fig. 5, n). Each nephridium 

 consists of a curved tube starting from the nephridiopore, passing outside the 

 transverse septum, and opening through two ciliated funnels into the posterior 

 part of the body-cavity. These organs thus belong essentially to the posterior 

 section of the body. These nephridia, which in structure may be compared 

 to the segmental organs of the Annelida, arise, according to Caldwell, through 

 the metamorphosis of the larval nephridia discovered by him in the Actino- 

 trocha. The latter, in their structure, recall rather the head-kidney of the 

 Annelida. They are paired canals which open externally behind the transverse 

 septum or diaphragm on either side of the invaginated ventral sac, the inner 

 blind ends being connected with a number of excretory cells. These are star- 

 shaped, a fine canal leading from each of them into the common duct. "With 

 regard to the origin of this larval kidney, Caldwell thinks that we may 

 regard the paired ducts as the remains of the communication established between 

 the posterior coelomic sacs and the surrounding medium by means of the pit- 

 like depression mentioned above (Fig. 2 C, g). The excretory cells, on the other 

 hand, would have an independent origin in the somatic mesoderm-cells. 



Phoronis is specially distinguished by the possession of a closed blood-vascular 

 system which ramifies on the intestine and in the tentacles, and in which a 

 fluid containing red blood-corpuscles circulates. The details of the origin of 

 this blood-vascular system are as yet not known, but it appears that the vessels 

 originate by dehiscence in the splanchnic layer of the mesoderm. While, 

 according to Cori, the vascular system of the adult is completely closed, there 

 seems, in the larva, to be a communication between it and the cephalic part of 

 the body-cavity. In the latter, the blood-corpuscles are said to arise in large 

 agglomerations. 



III. General Considerations. 



The Actinotrocha may without difficulty be regarded as a some- 

 what modified Trochophore. Indications are found in it of a pre-oral 

 and of a post-oral ciliated ring, the latter being transformed into a 

 row of tentacles, and of the characteristic neural plate. The 

 development of the mesoderm is of special importance. This is 

 arranged as two pairs of coelomic sacs which, however, do not 

 appear to be fully equivalent to one another, since, according to 

 Caldwell, they differ in their origin. The anterior pair of coelomic 

 sacs yields the cephalic cavity (lophophoral cavity),* the posterior 

 pair the whole body-cavity of the adult trunk. A transverse 

 diaphragm dividing the two parts of the body-cavity is found in 

 the Actinotrocha on a level with the crown of tentacles. The 

 nephridia belong to the posterior coelomic section. 



We can thus distinguish, in the body of the Actinotrocha, as well 



* [In the light of Masterman's observations this point requires reinvesti- 

 gation. — Ed.] 



