PHORONIS ARCHITECTA— BROOKS AND COWLES. 107 



Kowalevsky (11), Cori (-1), and Torrey (22) have all found the nerve ganglion, while Benham 

 (2) denies its existence in /'. auxtralix. It undoubtedly exists in P. architecta, is situated at the 

 base of the anal papilla between the anus and the mouth, and lies above the nerve cord between 

 the anal papilla and the nerve ring- (fig. 84). 



The ganglion consists of nerve fibres and nerve cells and the latter have at least two 

 processes. While it is a definite structure back of the anal papilla, on the sides it diminishes in 

 size until its cells become indistinguishable from those of the nerve ring. in fact, all of the 

 ectoderm forming the sides of the groove between the anal papilla and the base of the lophophore 

 is rich in nerve fibres and cells. 



The nerve ring follows the base of the lophophore on the outer side throughout its extent, 

 and in the inner part of the horseshoe it is quite rich in nerve cells whose processes can be seen 

 penetrating into the mass of fibres (figs. 67 to 74). This ring represents the collar nerve ring of 

 the Actinotrocha. 



There is a definite tract of nervous tissue running up the inner side of the tentacle, but we 

 are not prepared to say that it is a nerve running from the ring, although it is nervous tissue 

 which is undoubtedly continuous with that of the nerve ring. 



Cori (-4) has carefully studied the anatomy of the lophophoral oi'gan of /'. pxanimophUa and 

 we have nothing to add to his description at present. We are also unprepared to say whether or 

 not the second layer of the lophophoral organ consists of nerve cells. As he has described, they 

 have long prolongations which extend from the cells of the inner layer to the outer, and these 

 processes form a rather marked layer just below the epidermis on the outer surface of the organ. 

 At the base of the lophophoral organ these prolongations seem to be intimately associated with 

 nerve fibres which can be traced to the nerve ring. 



Throughout the body wall of the trunk there is a subepidermal layer of nervous tissue. 



Nephridia. — We have nothing new to add concerning the adult nephridia, but our observations 

 on 1'. architecta confirm those of Benham {•!) for /'. australis. The nephridial canals lie embedded 

 in the ectodermal wall in the region of the rectum. Each opens to the exterior through a pore 

 at the side of the anal papilla. Following the canal from the nephridial pore, we see that it 

 passes downward — i. e., posteriorly — for a short distance and then bends upon itself running 

 upward parallel to the descending arm. A short distance above the bend it opens by one funnel 

 into the lateral body cavity (fig. 72) and by another into the rectal body cavity (fig. 70). 



Reproductive organs. — Ikeda's recent paper (10) on the reproductive organs of Phoronis 

 gives a good account of the anatomy and development so we shall not enter into a description of 

 them. We are able to confirm Andrews's observations that the male organs develop at a different 

 time from those of the female. 



OiUated 'ridge of the alimentary canal. — Andrews has described a ridge running along the 

 inner wall of the oral branch of the alimentary canal (tig. 81). H. B. Torrey has found the 

 same structure in /'. pacifica, and we can confirm Andrews's observation for P. architecta. This 

 ridge does not seem to have any rudiment in the Actinotrocha, and it is not present just after 

 metamorphosis. 



SUMMARY. 



The male and female reproductive organs do not develop at the same time in P. architecta, 

 and the indications are that it is a protandrous animal. 



Fertilization is external and the eggs are not held in lophophoral masses by the tentacles. 



Segmentation is holoblastic and equal, but cleavage does not occur simultaneously in all the 

 blastomeres. During the division of the four-cell stage into the eight-cell stage, the upper four 

 blastomeres rotate in the direction of the hands of a watch. The sixteen-cell stage arises from 

 the eight-cell stage by a meridional division of each blastomere. 



The blastopore is eccentric from the beginning of gastrulation and the ganglion of the 

 Actinotrocha makes its appearance at this time. As development proceeds, the blastopore gradu- 

 ally closes up from the posterior end toward the anterior end of the larva until finally it becomes 

 a transverse slit. 



89369°— vol 10—11 S 



