444 



THE POLYZOA. 



rat 



=- -in.br. 



larval forms, methods of metamorphosis, egg formation, budding, and degenera- 

 tion. It is not our purpose to discuss these interesting but exceedingly intricate 

 processes; we merely wish to point out what we consider to be the chief morpho- 

 logical features of this particular division of the acraniates. 



One of the controlling factors in the morphology of the ectoprocta is an ex- 

 treme exaggeration of those initial defects in germinal material that have been 

 observed in all the acraniates. The result is that the first generation of zoids are, 

 at the very outset, unusually deficient in the various organs that are normally pres- 

 ent at those periods. These defective embryos fail to develop beyond the larval 

 stages, as practically all their organs undergo degenerative histolysis. After the 

 attachment, practically nothing is left of the original larva but a shapeless sac filled 

 with a mass of indifferent cells. A second factor in their morphology is that a new 

 generation of zoids arises from a bud-like infolding on the haemal surface of the 

 first larva. This new product is not to be regarded as the completion of the 



development of the first zoid, but as a 

 new zoid, representing a second genera- 

 tion that arises from the formless rem- 

 nants of the first. But while the first 

 generation of larvae conform in the main, 

 as far as they go, with the larvae of the 

 entoprocta, the second generation de- 

 velop into a new type, due to the fact that 

 its primary, or oro-anal axis, is bent 

 double, in exactly the opposite direction 

 from that in the entoprocta. That is, 

 in the ectoprocta the body is bent so as 

 to bring the caudal end to a point just in 

 front of the head instead of behind it, 

 elongating and making convex the 

 primitive neural surface, and greatly shortening the haemal surface, thus 

 reversing the conditions in the entoprocta. (Compare Figs. 301 and 303.) 

 In the ectoprocta, the resulting elongation of the body is therefore in a haemo-neu- 

 ral direction, and it appears to be brought about in the same manner as in Phoronis 

 (Fig. 305), by the evagination of the middle section of the enteron through the 

 space between the divergent, postoral nerve cords. The latter, owing to the very 

 unequal growth of the haemal and neural surfaces, appear to be transferred to the 

 haemal surface, but in reality undergo but little change of position. The primitive, 

 preoral ganglion, or forebrain, apparently fails to develop, or it subsequently atro- 

 phies, as in the entoprocta and phoronida. Consequently the so-called circum- 

 oral nerve ring of the ectoprocta does not represent the original nerve ring connect- 

 ing the pre- and postoral ganglia, but the ventral cords and their transverse com- 

 missures. (Fig. 303, B.} 



The lophophore on this interpretation may be regarded either as a single pair 



FIG. 303.- Diagrams of an ectoproctous polyzoan. 

 A, Seen from side; B, from neural surface. 



