446 THE PHORONIDA. 



ically they are better designated as either cephalic and caudal, or anterior and 

 posterior, corresponding approximately with the rostrum and carina of the 

 cirripeds. 



The stomodseum arises at a comparatively late period and is without doubt 

 formed on the primitive neural surface, as indicated by the location of the nerve 

 ring itself and by the position of the simple sac-like heart on the opposite side of 

 the enteron, h. 



The genital cells are located in the walls of the mantle, one pair in the cephalic, 

 g.o. 1 , the other, g.o. 2 , in the caudal lobe, recalling the arrangement of ovaries and 

 testis in the cirripeds. 



The nervous system, in spite of the relatively large size of the animals, is 

 rudimentary in the extreme, the central portion consisting of a slender circumoral 

 ring with a small, postoral ganglion. 



The forebrain, considered as a nerve center, may be regarded as practically 

 absent, since the preoral portion of the ring consists of little more than a slender 

 commissure. The fact is significant, in view of the absence of the preoral ganglion 

 in the polyzoa and phoronida owing to its histolytic degeneration during the 

 metamorphosis, and in view of the extremely rudimentary condition of the fore- 

 brain in all other members of the acraniates. 



VIII. THE PHORONIDA. 



In the ectoprocta we can only infer from the condition that obtains in the 

 adult, that the metamorphosis of the second generation of zoids takes place in the 

 manner described above, for the embryonic processes by which this result is 

 attained are not clearly defined. In the phoronida, however, where the adult 

 condition is apparently very similar to that in the ectoprocta, the successive steps 

 in the metamorphosis of the larvae are sharply differentiated and are sufficiently 

 well known to supply this deficiency. 



The eggs are nearly yolk free and undergo the early stages of development in 

 the recesses of the lophophore. The cleavage is nearly equal, forming a hollow, 

 nearly spherical blastula that is infolded at one side to form, apparently, a nearly 

 typical gastrula (Fig. 305, A), the blastopore remaining open as the mouth, and the 

 infolded cells forming the permanent enteron. The latter extends in a caudal 

 direction, the anus forming at the point where it unites with the ectoderm. The 

 mesoderm arises as isolated cells at various points from the walls of the blasto- 

 derm and especially in the vicinity of the anus. It is, apparently, not formed in 

 conjunction with the endoderm, nor does it separate from the later as definitely 

 formed ccelomic chambers. 



There is a prominent apical plate, a.pl., that probably represents the begin- 

 ning of the forebrain, or supra-oesophageal ganglion. 



A thick, longitudinal band, ab, is formed at an early period, that contains 



